WebGameDev

Options for developing games for web browsers using Common Lisp tools:

Flash:
Flash is probably the most widely usable option*, but doesn't work on iphone or ipad, and seems to be unpopular with lispers. It handles 2d fairly well, but only has rather convoluted support for 3d in current versions (10.1 and under).
 * 3bil is a CL hosted cross compiler that outputs bytecode for flash 9+ virtual machine, which together with 3b-swf (a library for manipulationg .swf files) allows developing flash games from lisp.

Both are still fairly rough at this stage, with lots of missing bits, APIs still changing, etc. 3bil is intended to resemble CL as much as possible, but currently lacks major features like packages, dynamic binding, clos, etc.

HTML/JS:
Writing games directly for the browser is becoming a more viable option, but still has some major limitations like lack of full screen support, and inability to grab input focus (which prevents things like usable mouselook, etc)
 * parenscript is a CL hosted cross compiler which compiles to javascript. Parenscript is evolving towards CL, but there is a fork from an older version of parenscript which is closer to a sexp version of JS.
 * Psos, adds a clos-like layer for parenscript.
 * slime proxy + clws allows using SLIME to work interactively with parenscript code running on a browser.

At the browser level, game style graphics is usually handled by the tag, which handles 2d graphics, and on on more recent browsers includes WebGL for 3d. 2d canvas is fairly widely available on non-IE browsers, and 3d support is available in beta for most browsers.
 * 2d canvas tutorial
 * webgl tutorials/info