The extensive library of extra Maxis for Simcity 4 game files, updates, patches, tools and maps, can be found at Simtropolis:

Maxis Files for SimCity 4

All official files remain the intellectual property of Maxis / Electronic Arts (EA). They have been made available for the benefit of the SimCity 4 community, following the closure of the official SC4 website (simcity.ea.com) where these files were originally hosted. By downloading, you hereby agree to the terms set forth in the Maxis End User License Agreement (EULA).