Afloat from Infinite Labs was one of my favorite SIMBL plugins, until it was broken by Leopard. It adds some clever features to windows in Cocoa applications, including a mode where the window remains always visible, "floating" above all other windows, an overlay mode where the window floats but clicks fall through to the windows below, and adjustable transparency (including a very nifty mode in which a window will be translucent until you mouse over it and then it pops into an opaque mode).
Thankfully, Afloat 2.0 now works with Leopard. In fact, it works so well with Leopard that OS X 10.5 is now required to run Afloat. The free download includes the Infinite Labs PlugSuit SIMBL plugin manager, but it is not necessary to run Afloat if you already have SIMBL installed.
[via Infinite Loop]