ResidualVM started as a sub-project of ScummVM for the "residual" LucasArts games that could not be added to ScummVM because they are 3D games, namely Grim Fandango and Escape from Monkey Island, using the Lua-based Grim Engine (GrimE).
It has gained its independence since then as a project similar to ScummVM but for 3D adventure games with several engines from other game companies being developed on.
For more detailed information on the current status please read the ResidualVM Wiki.