Win32 Constants Library by Tuncay: Look up all those "magic numbers" you need to use with PostMessage and SendMessage.
WM_NCHitTest by Andreone: Find out what's under your mouse cursor.
ManyTetris by Icarus: A Tetris clone with easily customizable templates to define game pieces and variables.
Connect 4 by Pekoe: A well-done AHK version of the classic game. The AI in this one is quite good.
Scriptlet Library by Toralf.: Store and organize all those small functions and snippets of code that you reuse over and over in different scripts.
AHK Window Info by Toralf.: A replacement for the AutoIt3 Window Spy currently bundled with AutoHotkey.
Enumerate Processes by Shimanov.: Similar to the Windows Task Manager, provides a list of running processes as well as their command lines.
Isense by Majkinetor.: AHK scripting monitor and syntax helper, an improvement on Rajat's "IntelliSense" currently bundled with AutoHotkey.