There are LOTs of mods for this game. Biggest places to get them are below:
User-Made Car Downloads:GTAzz Car PortalFantastic place to get new cars for GTA1 & 2. Over 5000 at the last count ranging from the very first in '97 right up to the sleekest modern creations!
File Dumps:GTA Gaming ArmyExcellent place for getting
dozens of GTA1 & 2 multiplayer and single player maps. A bit out of date though.
User-Made Cities:Scumbag CityBy our resident Super Admin Stilton, this city is generally regarded as having the most ambitious code and most detailed map out there. Despite the dynamic non-linear storylines stretching the abilities of the mission code, this level is remarkably stable!
You'll want to screw the missions up just to see what imaginitive chore you're sent on to make up for it!
SOMEWhereAzz's 18 month map. Critiscised by a few (namely me) for not having the extreme attention to detail of maps like Scumbag City, this map nevertheless is very playable and is graphically one of the best.
Delfi's SiteLong-time guru of GTA1 modding, Delfi has created countless utilities for GTA (including the infamous TXDGold - don't say the oldbies don't love you!

) including a couple of cities.
His trademarks are immaculatly presented maps with adventurous texturing and imaginative buildings... although the building techniques he uses are a bit hurried at times!
Cerbera's GTA1 MapsMy own forays into GTA1 modding left me with 1 finished city, called Cerbera2 with the almost unique feature of a bonus chapter, a half-complete one called Uphold the Law where you play as a cop, Pacific City which is widely regarded as having the most original map although no missions were made for it and the GTA: GT project which fell through.
I've always tried to make my mission code 100% indestructable, make my maps as neat as possible and do new things with the mission code. Judge for yourselves.