OpenGTA2

OpenGTA2 is a remake of Grand Theft Auto and Grand Theft Auto 2. It is based on a custom top-down game engine called OpenGBH, and uses different file formats than GTA 2 and GTA - although there are conversion tools which make maps and graphics compatible with OpenGTA2.

It is still in active development by Black Phoenix. Source is freely available at the OpenGTA2 SVN, and the engine itself is completely free, but the tools usage is restricted to non-commercial use, and you need original GTA2/GTA files (since OpenGTA2 is actually set of tools for OpenGBH).