In addition to the AutoIt authors already mentioned:
Robert Yaklin: A lot of tireless testing to isolate bugs, a great first draft of the installer, as well as great suggestions for how commands ought to work :)
Jason Payam Ahdoot: For suggesting and describing floating point support.
Jay D. Novak: For discovering many Win9x problems with the Send command, CapsLock, and hotkey modifiers; and for generously sharing his wealth of code and wisdom for hotkeys, hot-strings, hook usage, and typing acceleration.
Rajat: For creating stylish replacements for the original AHK icons; a great product logo; making the syntax customizations for TextPad; discovering some bugs with the registry commands and AutoIt v2 compatibility; making SmartGUI Creator; and many other things.
Thaddeus Beck (Beardboy): For NT4 testing to fix GetKeyState and the Send command; for a lot of help on the forum; and for many suggestions and bug reports.
Gregory F. Hogg of Hogg's Software: For writing the source code for multi-monitor support in the SysGet command.
Aurelian Maga: For writing the source code for ImageSearch and the faster PixelSearch.
Joost Mulders: Whose source code provided the foundation for expressions.
Laszlo Hars: For advice about data structures and algorithms, which helped greatly speed up arrays and dynamic variables.
Marcus Sonntag (Ultra): For the research, design, coding, and testing of DllCall.
Gena Shimanovich: For debugging and coding assistance; and for some advanced prototype scripts upon which future features may be based.
Eric Morin (numEric): For advice on mathematics; for steadfast debugging of areas like OnMessage, floating point computations, and mouse movement; and for improvements to overall quality.
Philip Hazel: For Perl-Compatible Regular Expressions (PCRE).
Philippe Lhoste (PhiLho): For tireless moderation and support in the forum, RegEx advice and testing, syntax and design ideas, and many other things.
John Biederman: For greatly improving the presentation and ergonomics of the documentation.
Jonathan Rennison (JGR): For developing RegisterCallback(), and for beneficial suggestions.
Steve Gray (Lexikos): For developing dynamic function calling and other new functionity; analyzing and fixing bugs; and valuable support and advice for hundreds of individual visitors at the forum.
jackeiku: For developing Unicode support and other new functionality.
fincs: For developing native 64-bit support and try/catch/throw, writing a replacement for the old compiler, analyzing and fixing bugs.
Sean: For developing built-in COM functionality and providing valuable insight into COM.
TheGood: For merging the documentation and adapting the installer script.
ac: For developing #Warn.
Russell Davis: For developing A_PriorKey, ahk_path (the basis of ahk_exe in WinTitle parameters), #InputLevel and SendLevel.
Christian Sander: For developing support for SysLink controls.
And to everyone else who's contributed or sent in bug reports or suggestions: Thanks!