MESS is an open source project which documents the hardware for a wide variety of (mostly vintage) computers, video game consoles, and calculators through software emulation, as MAME does for arcade games. As a nice side effect to this documentation, MESS allows software and games for these hardware platforms to be run on modern PCs.
As of version 0.138, MESS supports 479 unique systems with 1,282 total system variations and is growing all the time. However, not all of the systems in MESS are functional; many are incomplete.
MESS is in a constant state of development, as such, it should probably not be deemed stable, and used for testing purposes only.
MESS is notoriously difficult to learn to use, like it's MAME counterpart, it is commandline based, however there are front-ends available for easier usage.
To use MESS you usually need to obtain the BIOS rom dumps for the appropriate system.
The official homepage for MESS is mess.redump.net.