mod_gfx is a configurable Apache module that uses libgd to perform image operations on the fly. Some of the supported image operations are: resize, resample, watermark, and crop. A couple of its special features are origin selection via URI regex and round-robin over a list of hosts.
mod_mp3 turns apache into your basic RIAA hating, but every college student loving mp3 streaming server.
mod_musicindex is an Apache module aimed at being a C implementation of the
Perl module Apache::MP3.
mod_video lets you grab images from a frame-grabber card using the video4linux API.