TinyMUCK and its varieties is the software behind online systems such as FurryMUCK, Tapestries, and FluffMUCK. It is basically an Internet server that players can connect to, build rooms and objects, and program in MUF, a programming language built into TinyMUCK.
Several varieties of TinyMUCK exist; the most popular seems to be FuzzBall MUCK, which is the original TinyMUCK greatly improved by Revar. This version is the one that powers FurryMUCK. Other varieties include NeonMUCK, GlowMUCK and ProtoMUCK.