OpenGTA2
OpenGTA2 is a remake of GTA2 and GTA1 games. It is based on a custom top-down game engine called OpenGBH, and uses different file formats than GTA2/GTA1 - although there are conversion tools which make maps and graphics compatible with OpenGTA2.
It is still in active developement by Black Phoenix. Source is freely available at the OpenGTA2 SVN, and the engine itself is completly free, but the tools usage is restricted to non-commerical use, and you need original GTA2/GTA1 files (since OpenGTA2 is actually set of tools for OpenGBH).