If I listed all the software utilities I use, it would probably flow over into a second or third forum page. So here are just a few of the gems, in no particular order.
Note: I define a "utility" as a software product that accomplishes a specific task, or a very limited range of tasks--a "one-trick pony" as it were--and not as a 55-MB software suite that takes 20 minutes to load and causes at least one system freeze per week.
I also tried to list some of the more obscure utilities I have and use, rather than the same old ones that people never stop talking about.
CamStudio (freeware)
Create video (AVI) or Shockwave (SWF) files based on screen activity (awesome for reporting bugs)
HandyThing (shareware)
Make application windows and dialogs move to the size and position of
your choosing
AtYourService (shareware)
Run almost any executable or batch file as a Windows service
ColorCop (freeware)
Color picker with eyedropper color selection and screen magnifier
ERUNT (freeware)
Easily backup, restore, and compact the Windows 2000 or Windows XP registry
(I have a batch file for use with ERUNT that automatically creates backup folders named with the date and time, if anyone is interested)
Hotmail Popper (freeware)
Check or send mail with your Hotmail account, using any POP3-compliant email client
DumpSec (freeware)
Extract security and audit information from the file system, registry, printers, and network shares
Miranda IM (freeware)
Awesome multi-protocol open-source IM program, which supports a slew of plugins of all types
Total Uninstall (freeware)
Monitor software installtion and system changes, for later removal
NirSoft Utilities (freeware)
Too many freeware utilities to list, created by a guy who apparently never sleeps (my favorite: ZipInstaller)
Hyper Hasher (shareware)
Nice GUI utility to calculate file checksums and hashes, and for comparing files
WinHex (shareware)
Powerful utility for hex editing, disk editing, RAM editing, data recovery, disk cloning, etcetera
IP2 (freeware)
(Of course you do!) Easily determine your internal and external IP address (great for router users)
Instant Version Info (freeware)
Shell extension that lets you right click files to quickly determine manufacturer, file version, and other information
PowerZip (shareware)
Little-known but elegantly-designed archiving utility
Push The Freakin' Button (freeware)
Automation utility to dismiss annoying dialogs
Monitor Off (freeware)
Allows you to put your monitor in sleep mode programmatically
FreeMeter and FreeMeter Pro (freeware/shareware)
Displays system stats in real time, with an elegant interface
Hotfix Reporter (freeware)
Lists and provides links for any Windows hotfixes that are not installed (or may be questionable)
BOClean (commercial software; no demo available)
Excellent and constantly-updated real-time trojan scanner
PTBSync (freeware)
Highly customizable system tray clock replacement with atomic clock synchronization
SoftLeds (shareware)
Put hard disk read/write LED lights in your system tray, or right on your screen (OSD)
QuickMix (freeware)
Set your volume with shortcuts or batch files, or even with Task Scheduler
PurgeIE Pro (shareware)
Clean up after Internet Explorer and other messy applications the
right way
BootIt NG (shareware)
Reliable disk partitioning and imaging (and still the
only imaging utility that does true byte-for-byte verifications)
WhyReboot (freeware)
Determine why that stupid installer wants you to reboot
againEraser (freeware)
Secure file deletion and disk wiping
Resplendent Registrar (shareware)
Registrar Lite's big brother; a powerful registry editor that can prompt you before every change, and undo every change
Prime95 (freeware)
The
best way to determine if your system is truly reliable, period
UltraEdit (shareware)
The best text editor available, with a built-in FTP client
Discover (freeware)
Force dialogs and windows to remain on screen (great for people who change resolution often)