iTunes 10 is a huge pet peeve for me. The Windows version is such a huge resource hog. As soon as I open it, my GPU clock speeds spike for a few minutes. The same thing happens if I switch over to the iTunes store or the other major pages. I used the cover flow in iTunes 9, and my GPU hardly noticed. Of course, Apple makes it terribly difficult to tell them of iTunes shortcomings.
 
Anyway, aside from that:
 
Norton Internet Security 2011
Google Picasa 3
Firefox 4
TuneUp Utilities 2011
Acronis TrueImage Home 2011
MalwareBytes Anti-Malware
Steam
Skype
EVGA E-LEET
nVidia System Monitor
If you use Windows Sidebar Gadgets, check out Home Cooked Gadgets . The Custom Calendar and Drive Manager are great.
 
Oh, and I saw Ninite was mentioned on here too. Great if you're starting from scratch, saves a lot of time.

Icecat - Open Source Browser, secure and stable (9)
Truecrypt - Heavy heavy heavy encryption (10)
Libre Office -  Office Suite (7)
Gimp - Image Editor (8)
Brasero - CD/DVD Burner (9)
VLC - Media Player (9)
Audacity - Audio Editor (6)
Kdenlive - Video Editor (8)
Hardinfo - Similar to CPUZ and GPUZ combined and a different layout (9)
Unetbootin - Write an .iso to a flash drive and make it bootable (9)
 

I have a program that can dump the entire list of installed program, but a lazy administrator will not let me post the URL.
It took 3 months of programming in C++ to get it up.
License is very restrictive, my threads only, no other use allowed.
 

firefox 3
qq messenger
foobar2000
mplayer ww
steam
 

Google Chrome
Steam
MSE 
VLC
Winrar
ImgBurn

no particular order, the only apps I regularly use.
1. Fubar2000 for music
2. VLC media player, for ALL videos
3. Google Chrome
4. uTorrent
5. Steam for games duh
5. Origin for BF3
6. Speedfan
7. Rainmeter
8. MSI Afterburner
9. Logitech Setpoint
10. Teamspeak and Ventrilo
11. Auslogic Disk Defrag
12. CCleaner
 

I'll bite.  These programs are all critically important for a non-gaming computer ;)
 

• firefox + zotero + adblock/fanboy - track papers/citations and make sure that you don't get annoyed by advertisements and can block porn-esque domains at work (on gawker/deadspin sites, sometimes the jezebel article profile images are NSFW)
• chrome/opera/safari (doesn't hurt to have 'em all) - recently (Mar 2012) needed chrome when IE and firefox failed due to a corrupt .dll from an MSUpdate gone bad.  Also handy for any web developers out there to make sure display is consistent across browsers.
• thunderbird - e-mail more secure than Outlook.  Free
• endnote (zotero may be enough by itself) - citations, etc
• antivirus-of-your-choice + anti-other-baddies - goes without saying
• Adobe Acrobat - pdfs are commonplace where I come from
• bullzip pdf printer - make pdfs when other people don't have open/libreoffice (below) and you have images in your documents
• libreoffice - like MSOffice but free.  Bugs I've found are largely restricted to the database program, but pages with multiple images when saved in MSOffice format overlay rather than sit nicely next to each other.
• coretemp - make sure you're not overheating, even when doing something silly like compressing a file
• cpuz - see if your cpu is stepping up/down in response to load, look at other simple system issues 
• gpuz - check on GPU RAM load, see clock speeds and other diagnostic info about your graphics card
• WinSCP - like FTP, but secure .  supports ftp, scp an sftp (and possibly other protocols too)
• putty - connect to a remote (typically linux/mac) computer
• whocrashed  - diagnose system crash info...give generally helpful info
• komodo - programming IDE/GUI
• python - simple programming language that you can run locally
• pdfsam  (split and merge) - if you want 1 page from one document and want to combine it with another PDF (say you scanned in a signature and need to attach it to the rest of the document that wasn't scanned)
• process explorer  (sysinternals) - see what programs are loading what DLLs, what directory they're running from and a host of other things that the windows task manager either can't handle or doesn't by default.  Can also hide/show windows
• skype - chat with your face
• gimp2 - free photoshop!
• ghosttools/ghostview - postscript files
• 7-zip - best general compression program available for the PC.  Can handle gz and bz2 archives in addition to the typical ones
• NetSetMan - a lifesaver on a laptop that goes between work and home.
• mDesktop- helps reduce clutter at any one time, allows you to divide-by-task.
  
• cygwin - emulate a linux environment....I can't always get this one to do what I want though
• ubuntu or your choice of linux OS on a partition - good to have an alternate OS from time to time in case you get hit by a virus or have other issues.  Also allows for a platform to run clonezilla or partimage or some other backup program for your windows bits.

In the order I put them on after fresh install:
1. Firefox
2. Avira Anti-Virus
3. Some PDF printer
4. Some screen grabber
5. WinRAR
6. Emacs
7. ...

+ Open Office, of course.

sequoia view, vlc, putty, filezilla, ccleaner, notepad++, djv_view, precision, eleet, renamer, prime95, 7zip