Zombie Nation MUD (ZNMud) is a Multi-User Dungeon engine. It is based on the Teensey engine for its lower level socket handling. It was designed to play out zombie outbreak ideas but can easily be expanded to host other concepts. It's a massive multi-player text-based game (although it does support MXP). It also supports modern day weapons, etc.
libtelnet is a small library for writing telnet protocol handlers in C. It uses a fully correct state machine model and full RFC 1143 (Q Method) option negotiation. It includes optional support for MUD-specific protocol extensions such as MCCP2 and ZMP. Several test programs are included, such as the very useful telnet-proxy utility.