Never thought about modding until you mentioned it in a post a few months ago,so i had a little dabble.
GTA3... Played as spiderman and ronald macdonald (sad,but easy to start with)and a few different vehicles and buildings.
Mafia... Got extra vehicles.
Just a dabble to get an idea of whats involved. May well try and mod a game properly when i find a suitable newish game on vista.