(SC3ku)
Simstuff Hall - A great site with a few customs for both SimCity and The Sims. Also has an Aimoo forum.
Webmaster: Buildingmaster01/Walgr
Freeland - Has hundreds of customs created by several different people. Has extensive fansite page. Forum and chat. Webmaster: Fredfree
(SC4)
Sorry, I haven't found any SC4 fansites yet, but I am looking.
(TS)
Spiffworth Network - A site that is currently under construction but I think that it will be very great when finished.
Webmaster: AdamL [Spiff/Bruntsfield]