Right now the only mud I play is Multi Users in Middle Earth (MUME). You can check it out at mume.pvv.org, lots more fun than I thought it would be. Also, being a programmer, I ended up writing a lot of powwow scripts to help me play, so if you're looking for examples of powwow scripts for MUME, go to my MUME page.
The latest version of powwow is also hosted here, which includes plugin support, bugfixes and other feature enhancements since 1.2.5.
Since we have an AS/400 where I work, I also thought it'd be fun to try and MUME from a greenscreen terminal. You can find pics about that here