Cocktail 4.0 is out and brings to this system utility a host of new features. The biggest item in Cocktail 4.0 is Leopard support. 4.0 also adds Sparkle updating, new Automator actions, better help, and lots of bug fixes.
Cocktail allows you to tweak a dizzying number of things about OS X. This is a must have tool for all you Mac geeks out there, and it is a steal at $14.95 for a single license.