LuminariMUD is a d20 and pathfinder systems hybrid based off recent releases of tbaMUD which has integrated a significant portion of code from the fantasy version of the d20mud codebase.  With a unique, and vibrant, living world, and some fantastic polish on the playing experience, it is a great implementation of d20/pathfinder rules.  It's under regular development with a decently sized team of active coders, builders and scripters.

Check out the LuminariMUD web site for more info or jump right in at port 4100.