So, I've tinkered with computers for years, and, like most I imagine, I've accumulated a number of odd, but useful, programs. Just odds and ends, a bit like the contents of your kitchen "Junk Drawer," bits and pieces that, taken individually, may or may not be useful, but with some knowledge and creativity can solve a wide variety of problems.
Here's some of my most used utilities/UI tweaks (on my main PC running Win7):
(attempts at prioritization failed, so this is no particular order)
1. Cygwin -- an assortment of Linux programs using a compatibility layer to run on Windows, this is a large toolbox several other items on this list are included in this
2. Send To -> Notepad -- open any file in notepad with a quick couple keystrokes
3. bash (Cygwin) -- why use CMD.EXE when bash does so much more? granted, as of Vista, CMD's much nicer, but bash sill beats it IMO
3. Pidgin -- multi-protocol IM client, works with Gtalk, AIM, MSN, even IRC. open-source. loads faster and uses less RAM than 2 or 3 individual programs
4. perl (Cygwin) -- perl... the ultimate (IMO) glue language. you'd probably be surprised at how much you can do with a perl "one-liner"
5. 7zip -- best compression program I've ever found, it can read every archive format I've come across (even ISOs!), and has some nice shell integration (right-click menu options to extract/compress etc.)
6. VirtualClone Drive -- small, effective ISO mounter
7. file (Cygwin) -- uses "Magic Numbers" to determine file type, for when filename extensions fail. it can also usually determine the codec used for a media file
8. MMConvert -- general purpose media converter. with the right codecs installed, it can convert just about any video or audio to another format
9. BitTorrent -- because I'm a shameless pirate
So, what utilities would you be lost without?
Let's leave out firewall/anti-virus software, they're more like a diver's O2 tank than his underwater camera.