1. RocketDock (freeware) - This is a sexy application! It does a really good job of simulating the Apple Dock feature. Mouse over any of the icons and they will smoothly zoom larger for launching. Add your own applications for launching. Customize the skins. Supports multi-monitors, alpha-blended ICO/PNG icons, transparency, auto-hide. It's super-customizable. I just wish that the developers would add a way to lock the horizontal size and allow the displayed list of items to be scrolled.
2. RK Launcher (freeware) - Oh, Baby! Kiss Me! Another very beautifully crafted application that simulates the Apple Dock feature. Here is a nice almost perfect OSX Tiger theme for RKLauncher you can use instead of the standard default one. The really nice feature of RK Launcher is that it unpacks to a folder and is "standalone" meaning it doesn't install into your system. That always gets very high marks from Claus. This could be a very sexy alternative to Windows Quick-Launch bars.
3. Samurize (freeware) - This tool provides extensive desktop enhancements for system monitoring; including meters for CPU utilization, network traffic, disk access, system temperature. In addition to clocks, weather docks, news feeds, and a ton of other widgets. It is very easy to get started. There are a wealth of skins. Once you get used to "programming" it, you can really go crazy.
4. Folder Marker (freeware) - With this freeware application for Windows, you can quickly apply custom colors to your folder icons, select up to 10 additional custom icons to apply to folders, and apply categories and sub-menus to your folder icon choices.
5. UberIcon (freeware) - from the makers of RocketDock, this application adds some more fun "effects" to your icons when you launch them, including iZoom, iBounce and FlatOut effects. All fun aside, the iZoom feature might be useful to some "older" users who might have trouble visualizing the icons on the desktop but don't want to crank up the icon size setting in Windows.
6. PlacesBar Editor (freeware) - Tired of being forced by Microsoft to use those "handy" commonly used locations when you are opening/saving a file? Mine were "My Recent Documents, Desktop, My Documents, My Computer, My Network Places." Only I don't use two of those, ever. Now I have Desktop, My Computer, My Documents, Downloads, and Claus Utilities! This mini-app allows you to select ANY folder and exchange it for ANY of those listed (although you must have just five). Nice!
7. Yahoo! Widgets (freeware) - Formerly known as Konfabulator. This application provides the user a choice from over a thousand specialized applications that can reside on the desktop. Most are very pretty, many are actually very useful. A (very) small sample include games, news feeds, picture frames that feed images to your desktop, calculators, notepads, web-mail checkers...the list could go on forever. Really polished widgets here.
BB4Win (BlackBox for Windows) (freeware) - If you like a lean and mean desktop "windows" manager, then you will likely love BB4Win. It is heavily reminiscent of FluxBox or BlackBox for Linux systems. Fully customizable. Light and fast. I've been able to run successfully on both Windows 2000 and XP systems. Also: bbLean (freeware). This one can run off a USB stick for portable usage--or use it on your system proper. UPDATE: Harmon kindly pointed out in the comments that BB4Win is no longer being supported. Somehow I missed that fine detail when I checked into the link. Try using bbLean and you should be just as happy. Comparisons to bbLean and the Linux ones noted still apply closely.
9. Glass2k (freeware) - make any window transparent. Simple? Yes. Cool? Oh yeah! Really handy if you run dual monitors and have multiple applications open, but you don't want to hide that wicked-pretty desktop wallpaper.
10. TweakUI (Microsoft freeware) - I know that probably most every XP/Windows2K user who has started customizing their desktop already has this Microsoft application already. Right? Nope? Oh my gosh! Go get it now! It is really almost a mandatory MS PowerToy enhancement that allows you easy access to desktop and system tweaks that would normally only be possible through registry hacks. Similar tool: XPredit.
11. Change XP Log On Screen Colors (registry hack) - Tired of that same baby-blue color on your XP login screen? Need something different? Did you know Microsoft actually has three default "shell styles" for XP Luna? Homestead, metallic, and NormalColor? It does!
12. Change your XP user account image (system tweak) - Don't feel a duck, frog, or chess piece really represents the true l33t skillz you have as a computer user? Don't settle for the pictures Microsoft gives you, make and set your own! Simply and quickly!
13. TweakXP.com (hacks, software and tips) - Tons of useful registry hacks, user tips, and software leads. For starters, since we are on the subject, start out with the User Interface Tweaks page. Explore the additional categories at your leisure.
14. Toss out the Start-Run method of launching your applications. Yes. I've put RocketDock at the top, but there are a slew of additional ones. Scott "Computer Zen" Hanselman guides you through the best.
15. Google Desktop (freeware) - Can't mess with Windows and not mention something from Google. Gadgets, clocks, meters, media player launcher, birthday reminder, sidebars, search enhancements, Google integrations. Whew! Do these guys ever sleep?
16. FlyakiteOSX v.3.5 (freeware) - I haven't tried this one yet, but it sounds pimp-a-licious cool!. When you click in, don't let the login screen fool you. Just enter any username you want and "log in". You will be treated to a gorgeous Flash environment. Use the menu bar at the top to select the page items to preview. This is amazing stuff! Note: This does some MAJOR system changes to transform you XP system to a truly complete OSX style. Only use if you are very brave! From the developer: "FlyakiteOSX is a transformation pack. It will transform the look of an ordinary Windows XP+ system to resemble the look of Mac OS X. The installer simply automates the process of replacing critical system files, setting registry tweaks, and installing extras such as cursors, sounds, visual styles, etc. In case you are not happy with the results of FlyakiteOSX, everything is completely removable. Just run the uninstaller from the Start Menu or from Add/Remove Programs."
18. Change your XP Boot Screen (freeware / hack) - (DANGER! Will Robinson! DANGER!) Don't mess with this unless you are just really a l33t system haxor. Back up you system. You can also try the freeware "Boot Editor" program. Read the tutorial. More here. See also LogonLoader (freeware).
20. Replace those wallpapers (free). Get your quality wallpapers from InterfaceLift, Wallpapers Zone (NSFW), KDE-Look.org, WallpaperStock, MyDesk Display, deviantART (dual/multi-display desktops!), Freddie's Wallpaper (Anime), AnimeWallpapers (Anime), PixelGirl Presents, and Digital Blasphemy.
Yes, I know I've left out sounds and cursor mods. Those aren't really my things to pimp in XP. I have some "custom" sound events set for startup, new mail, and shutdown sounds on my machines, but that's it. And I've cleaned way too many pc's at work off that had unauthorized versions of CometCursors. The standard Microsoft cursor offerings are just fine with me.
Have fun, just pimp with caution!