This is a mod which was originally started in 2008 by gtasearcher from GTAForums. It was dead for some time, but now I'm continuing it. You can find gtasearcher's mod page here and mine here. Continuation was done with gtasearcher's permission.
It improves GTA III's missions, mission rewards, adds ENB Series and much more! And remember to follow mod's official page on GTAGarage.com for the latest info. Current version is 1.08.