MMM Mode is an Emacs add-on package providing a minor mode that allows Multiple Major Modes to coexist in one buffer. It is particularly well-suited to editing embedded code or code that generates other code. It defines a general syntax—the "submode class"—for telling it how one major mode should be embedded in another.
The original author of MMM Mode (Michael Shulman) stopped maintaining it a long time ago. As of February 2013, development has moved to Steve Purcell's GitHub repository. The new release, compatible with Emacs 23 and 24, is also available for download at the old project page on SourceForge
There is also an MMM Mode mailing list hosted by SourceForge, although it hasn't seen much activity in years.