Similar to the Vice City Rage project (and the San Andreas Rage project), here's an undertaking bringing the world design of Grand Theft Auto III's Liberty City to the more robust engine of Grand Theft Auto IV.
Why bother? Same old classic game world you know and love, only now the cars drive better and the on-foot combat isn't an exercise in self-flagellation.
If you want to try it out, you can grab it at the link below.
Grand Theft Auto 3 On Rage Engine [GTA Forums, thanks Sebastian]