ColdMUD
ColdMUD is a type of MUD server software, similar in many ways to MOO and CoolMUD, from which it is descended. It is a text-based online virtual reality system to which multiple users are connected at the same time. Created by Greg Hudson, it has many similarities to both MOO and CoolMUD. However, there are many important differences from MOO in its embedded, domain-specific programming language, ColdC, and in its implementation of persistence and network services:
Like CoolMUD, but unlike MOO, ColdMUD's object database is fully disk-based. Whereas MOO's implementation of persistence is based on periodic "checkpoints" during which the complete state of the server's object memory is written to disk, CoolMUD keeps the majority of the object database on disk and keeps a cache of the most frequently used objects in memory, a strategy known as incremental persistence. The ColdMUD implementation of the disk-based object memory is based on Marcus Ranum's work from UnterMUD.
Unlike MOO, the embedded programming language (ColdC) can be used without using the runtime as a MUD (that is, network accessible) daemon.