MEPIS refers to a set of Linux distributions distributed as Live CDs which can be installed onto a hard disk. The most popular of these distributions is SimplyMEPIS. MEPIS distributions are based on Debian, some of them indirectly. For example, SimplyMEPIS 7.0 is based mainly on Debian, with additional source code from Ubuntu (which is also based on Debian).
From Windows to MEPIS (A Windows user's guide to MEPIS)
MEPISlovers, main community site
antiX: a fast, lightweight and easy to install Linux distribution live CD based on MEPIS