Simple DirectMedia Library
NEM - Aha! Someone has made a page for this. I'd better fill in details. The Simple DirectMedia Library is a cross-platform C library for dealing with graphics, sound, joystick and keyboard/mouse input. It provides a nice API for setting video modes and drawing graphics, and has some OpenGL support. This is great stuff for writing games. Currently, I am working on a simple binding of SDL for Tcl. This will be pretty much a direct mapping of the SDL API to Tcl (but following Tcl's conventions). This will be mostly useful for scripting SDL components written in C, but you could write stuff in Tcl if you can live with the performance hit. Kristoffer Lawson is working on a higher level binding for Tcl, which will probably be more useful for most games. My version will probably be finished quite quickly, as it is a simple binding. After that it should be easy to merge into Kristoffer's code to fill in the gaps.
Other game libraries it would be nice to have in Tcl:
OpenAL - a 3D sound library, useful for effects. I don't think Snack has spatial sound.