AutoHotkey is a free, GPL’d project for “automating almost anything by sending keystrokes and mouse clicks”. It has a very flexible scripting language that allows you to map combinations of keys and clicks to arbitrarily complex commands.

NiftyWindows is a very nice pre-done script for AutoHotkey that provides mouse-shortcuts for common window operations like move, resize, Alt-TAB switching etc.

[Added 2008-04-21] Easy Window Drag sample lets you easily drag windows around without having to hunt for the title bar. I slightly modified the script to let me ALT + Left mouse drag windows around, Linux-style :).