killer apps

Web Browser

Mozilla Firefox. Platitudes might never adaquately describe what a breathe of fresh air this app is. Netscape's decision to open source its code is now paying hefty dividends as Firefox demonstrates time and again to be secure and feature-rich. And if it doesn't have enough functionality built-in for you, there are scores of plug-ins written in XUL available. Firefox is a stable app with a large community supporting it. It's not going anywhere anytime soon.
Runner-up: Opera. Despite having even more built-in features than Firefox, it still maintains a small footprint. The banner ads displayed prominently in unregistered versions are a bit much, but nothing can reproach this browsers at what it does best, porn surfing.
Apps that will kill you: Internet Explorer. The other day while walking to work, I saw a dead mouse on the sidewalk. It was covered by flies. That mouse still wasn't as buggy as this app is. And the decaying mouse had more features.

Music Player

iTunes. This app has it all: fast search, a nice music store, podcasts and the ability to share music libraries on a LAN. It doesn't hurt that it also supports the best MP3 player of all time.
Runner-up: Winamp. This vanilla app is simple and efficient.
Apps that will kill you: Windows Media Player. Start up the application, it hangs. Switch to view your libarary, it hangs. Try to view all Rock music, it hangs. When programmers write apps like this, they ought to hang.

Video Player

Divx Player. Beautiful interface reminiscent of Apple design. It just plain works.
Runner-up: QuickTime. Ditto, but only with their file formats and slightly less useful -- typical Apple design.
Apps that will kill you:
Real. Buffering ... It's gotten better in ... Buffering 75% ... 90% ... recent years, but I still wouldn't suggest any more than casual use.
Windows Media Player. In typical Microsoft style, it's a gargantuan, bloated app that claims to do everything yet accomplishes nothing adaquately. And did I mention the ugly interface? All the skins in the world won't fix it.

Editor

Notepad. It does only one thing, and it does it well. Is it possible that a Microsoft engineer took a computer science course in his life?
Runners-up: WordPad. Occasionally, you have a file too large for Notepad, or you need to do some light formatting. WordPad is there for you.
Apps that will kill you: Word. And occasionally, you feel like having an uppity animated paper clip act like it can write a letter better than you can. Now that Microsoft is embracing an open XML-based standard for its Word documents, we can all find solace in the fact that cloning it is that much easier for the kind folks of the Open Office project.

Integrated Development Environment

Dev-C++. Open-source IDE with a clean interface, a pretty editor, and a port of GCC doing all the work underneath. What's not to like? A must-have for freshman CS majors taking classes still teaching C++.
Runners-up: GNU tools on Cygwin. The first thing a Unix hacker does when forced to use a Windows machine is install Cygwin. Not quite like home, but good enough to keep one's sanity.
Apps that will kill you: Visual Studio. Is it even ANSI-compliant yet? Ugh, don't get me started. And you can stick your .NET where the sun don't shine, too.

Instant Messaging

Google Talk. Its interface is clean and discoverable, including VOIP and GMail integration. This program is still in beta, but Google's technological might and devotion to open standards ensure that the best is yet to come.
Runners-up: ICQ. Among the first IM apps ever, ICQ remains a haven for serious hackers and serious idiots. /me respects it.
Gaim. This is the open-source IM app that could. Supporting multiple protocols isn't easy, but they're doing a darn good job. It also supports features only found elsewhere in proprietary add-ons to the triumverate of hellish proprietary IM apps. Which brings us to ...
Apps that will kill you: AOL Instant Messenger, MSN Messenger, Yahoo Messenger. The unholy trinity itself. They don't talk to each other; they suck; ads suck; and they suck. They personify vendor lock-in.

Archiving Utility

WinZip. Archiving is its business, and business is good. For as long as I can remember, WinZip has reigned supreme while other programs came and went with the wind. Even Microsoft's anti-competitive move to bundle an archiver into Windows hasn't toppled WinZip. In addition to simply zipping and unzipping, WinZip can decompress tar'd and gzip'd archives and has added great features like virus scanning and self-extracting archives.
Apps that will kill you: WinRAR. There are few things as fustrating as spending a couple of days downloading a large file only to find it scattered about in a few dozen r-dot-whatever files, finding oneself at the mercy of a piece of nagware. Will the warez community please get a clue and ditch this proprietary pain in the ass?

System Utilities

a.k.a. programs to fix your girlfriend's computer
Spybot - Search & Destroy and Ad Aware SE Personal. These two programs are tied for this category, as they work best together. They're number one in their field because they're free, easy to use and lifesaving. Few apps are as indispensible as these; don't run Windows without them.
Runner-up: Disk Defragmenter. As sad as it is that your hard drives are utterly unorderly and, well, fragmented, it's reassuring that there's an app out there to glue the pieces back together, speeding file access and maybe even freeing up some space.
Apps that will kill you: the Windows operating system. After all, if it were better, this whole category wouldn't be necessary, would it?

Small Game

Minesweeper. The one and the only. Still waiting for that Web page to load or that bloated software to initialze? Double-click that mine icon and this lightweight app fires up immediately. Additionally, the games are over just as quickly with expert games lasting no more than a few minutes. Even as your computer slows to a sloth's crawl because every other app is failing in spectacular fashion, Minesweeper is there for you. Best of all, Minesweeper has the qualities of a classic game: easy to learn and impossible to master.
Runner-up: Yahoo Games. Yahoo has cast its net incredibly wide, so they were bound to get something right. That something is Yahoo Games. It boasts a truly impressive range of games, many of which are capable of competition online that just plain works. And all of this in a Java applet in your browser. There's just nothing like kicking ass at pool, chess and poker across platforms and continents.
Apps that will kill you: WildTangent, PopCap. Alas, the state of so-called "free" games online usually end up costing ya. If they don't blatantly spy on you, they probably want twentysome bucks to play past the third level. No thanks.

Math

Maple. From freshman calculus to PhD thesis, Maple is the mathematics package. It knows more math than you ever will, and thanks to contributions from mathematicians everywhere, it's getting smarter all the time. And for those without math degrees, it has extensive help documents that truly help.
Runners-up: Windows's Calculator. It does anything you could reasonably expect from such a small app including calculations in binary and hexadecimal. It joins Notepad as the only Microsoft apps I know of that programmers can use every day without cursing it.
Google's Calculator. That all goes double for this calculator. No need to open up another app when the answer can be found through your browser's built-in search utility (which of course is set to Google). In addition to the basic functions you'd expect, this calculator supports conversions of just about every unit you could think of. How many milliseconds are in a century? Or how many teaspoons are in a gallon? Much more importantly, how many yen is a dollar? That's right, Google calculator supports currency conversions, too. As an added bonus, it can calculate "the answer to life, the universe, and everything" without waiting for seven and a half million years for Deep Thought to figure it out.
Apps that will kill you: MATLAB. Sure, it may be better for matricies, but you don't really want to do linear algebra, do you? The stilted and inconsistent syntax is just the tip of the ugly iceberg for this app.

Honorable Mention

Audacity. Excellent interface on top of a first-rate piece of audio editing software. A true testament to open source.
Nero. Does exactly what you tell it to do, burns CDs and DVDs. If you're lucky, it came bundled with your burner.
Finale. Composing music can be quite involved, but Finale makes it seem easy. It warms the heart to know that a well-designed piece of software can encourage creativity like this.
Google Desktop. The clearest evidence yet of Google's plans for world domination. It integrates many of the best Google products to date and welcomes add-ons. If Google can garner a large user following with these widgets, it may be serious competition to Windows Vista's cheap knock-off of Apple's Dashboard.

Notably missing

Image manipulation programs: I neither know nor care enough. Photoshop has a consistent and discoverable interface, but it takes forever to load -- far too long for my purposes which usually doesn't go beyond shrinking an image. I've never learned to use the GIMP, because c'mon, when have you ever been asked to edit images in Linux? Besides those two, there are no other apps I know of that aren't laughable (Paint Shop Pro? Give me a break).