@tech

Trillian :: Although it loads slowly due to having many features that you unfortunately can't disable, this is a reliable first choice for IM applications and allows you to communicate using all of the popular IM services.
Pidgin :: This open source software is a close second. I must admit I haven't used this since they changed the name and who knows what else, but when it was called Gaim, it was a good second choice that loaded quickly but just didn't seem as reliable as Trillian.

Email
Eudora :: I still like Eudora but it can take a long time to load which frustrates me these days. I see on their web site that they no longer have a paid version but the ad sponsored version is still available. The BIG news however is that apparently Mozilla, the makers of Firefox, will be releasing an open source version soon called Penelope, so I'll be anxious to test that!
Microsoft Outlook :: which of course comes bundled with Windows, is the preferred choice of many. I don't like it simply because it tries to take over all of your communications.
Pegasus :: is a strong and free contender which I have used at times. It's fairly powerful/flexible making it a little slow to load and it may have more options than many people need.
Scribe :: This is a recent discovery for me, and so far I really like it. It's available in a free or paid version. This is a fine program that's simple to use and really fast that's quite noteworthy.

Web Browser
Firefox 2 :: With tabbed browsing and freely available extensions you can download that add extra functionality, this in my opinion, is the best choice for a Windows web browser. It's MUCH more secure and faster than MS IE.
Opera :: is a good second choice and is now free.  This is the original tabbed browser and is very similar to Firefox.

Office/Productivity
OpenOffice :: This is an open source, full-blown office suite with word processor, spreadsheet, and more. It's a slow load, but still faster than MS Office, and offers a powerful suite of tools that rivals MS Office.
AbiWord :: A free word processor that will open all of the common document formats. Since it's only a word processor it loads quickly, allowing you to get in and get out without waiting on other functions to load.
ConTEXT and Notepad++ :: I group these two together because they're both excellent, simple text editors with similar functionality. There are some differences between them making one a better choice than the other based on your needs, however both are highly recommended.
WordWeb :: an excellent, free English thesaurus and dictionary that will also help you with checking your spelling.
Foxit Reader :: A free and very fast loading program that will do a great job of allowing you to view PDF files.

Security
AVG Anti-Virus Free Edition :: This is a solid anti-virus software that gets updated several times a week, keeping you protected from all the latest bugs.
ZoneAlarm Free :: I turn off the built-in MS firewall and use ZoneAlarm which offers better and more flexible intrusion blocking.
If you want to go beyond the basics mentioned above, the following are great for securing your PC: HijackThis, SpywareBlaster, and Spybot – Search & Destroy

Utilities
CCleaner :: A freeware system optimization and privacy tool. It removes unused files from your system – allowing Windows to run faster and freeing up valuable hard disk space.
Fresh UI :: A tool for configuring and optimizing Windows.
Startup Control Panel :: Allows you to easily configure which programs run when your computer starts.
xplorer2 :: This is a great Windows Explorer replacement that gives you a lot of functionality and a 2 pane window which makes file management much simpler.
PageDefrag :: This nifty program defragments your paging files, Registry hives, event log files, and Windows 2000/XP hibernation files (where system memory is saved when you hibernate a laptop) every time your start your computer.
Process Explorer :: A much more detailed replacement for the Windows Task Manager.
ZipGenius :: Compress files to almost any kind of archive.  A must for opening ZIP files.

Web Development

Games
Clickomania :: A free and super addictive block removal game, that at first appears to be another Tetris but isn't. If you tend to get trapped into wasting a lot of time on games, do yourself a favor and don't download this one, or you may never sleep again. Consider yourself warned.