Telnet Games is a free, lightweight flat-file MUD listing service that allows anonymous web contributers to add MUDs to a list of services. The services are polled on-demand and nightly for reliability, and after 30 days of consistent downtime, MUDs are moved from the active list to the The MUD Graveyard. For the first 90 days, MUDs in the Graveyard are checked nightly to see if they can be revived and placed on the main list.
The software behind this service is called Web Mud Lister and is distributed under the LGPL, as long as no GPL add-ons are used, otherwise it falls back to the latest, most compatible GPL version. It can be picked up at this link.
Injection protection is used to thwart baddies.