Just with the click of a button, 2DOrNot2D will switch the Dock's appearance in Mac OS X 10.5 "Leopard" from 2D to 3D or vice versa, thus replacing the need to open the Terminal application and typing in cryptic commands you'd have to remember.
It is inspired by a blog post by Michel Jansen (http://micheljansen.org/blog/entry/395?lan=english).