$20, Mac OS X 10.4 (not OS X 10.5)
ShapeShifter is a program that lets you radically change the appearance of Mac OS X right down to the core – windows, buttons, scroll bars, and pretty much everything else can be "skinned" using freely downloadable themes, some more outlandish than others. The original graphic elements are kept safe, too, so you can revert back at any time, but it's not recommended for the faint- hearted nevertheless.
$13, Mac OS X 10.4+
Although OS X is infinitely better than Windows when it comes to installs, apps can leave libraries, preferences and caches dotted around, so trashing the app isn't always enough. AppZapper tracks these rogue files and shows them to you so you can safely delete them. It's a good way to try out programs and delete them totally afterwards.
$10, Mac OS X 10.3.9 or 10.4 (not 10.5)
Windowshade X is one of the best third-party solutions for managing windows. A double-click can reduce a window to its title bar, Minimise in place makes the window tiny but viewable, and a third option renders the window almost transparent.
Freeware, Mac OS X 10.3.8+
TinkerTool allows you access to a large number of hidden preference settings already present in OS X but which Apple ordinarily hides away. You can customise the appearance and behaviour of Finder and the Dock, the format of screenshots, turn off Dashboard, and a host of other useful settings. It's all non-destructive, so you can easily reset the original settings.
$29, Mac OS X 10.3.9+
DragThing is a quick launcher which combines features from the Dock and Finder to provide quick access to documents, folders and applications. With multiple configurations and support for hotkeys, it stores frequently used items and lets you paste them into apps. It also allows application switching and the possibility to move the Trash bin to the Desktop.
Freeware, Mac OS X 10.4 , limited functionality on 10.5
Quicksilver is a slick add-on for OS X. At its simplest it's a launcher, providing a way to launch any program or file at a keystroke, but it's actually much more than that. You can install application-specific modules which give you new actions based on that app, plus the ability to upload, rename, move, open and manipulate items from its floating interface.