Okay, to put a collection of SC4-links together is probably not very new or original – however, since this game is a few years old, many links you find are 'dead' ... So I put those together that I have found to be still active and hope they can help you to enlarge your personal collection of custom content.
These are well proven communities with download areas, city journals, galleries and contests, as well as forums and tutorial sections. The figures vary, but from these sites alone you can access something between 4 and 6 Gb of custom content.
The languages include english, french, spanish, portuguese, german, polish, finnish ... but you'll find your way around quite easy, at worst you can use one of the online translators.
Now this next group is a little harder to handle, the sim-sites from around asia – mainly Japan and Korea. I admit, some of these links are not really precise, meaning they don't lead you straight into a download area. Instead you'll have to click around to find out where the real treasures are hidden. So these links are for the hardcore hunters, that bring along a little time and lots of patience. However, the rewards are cool – loads of fantastic lots and buildings, that cannot be found anywhere else.
Note: These asian sites are not really constant -neither in naming conventions nor in location- and frequently change or even go offline. I try to verify the links once in a while, but you may still encounter broken links...
In no particular order -
The best help you can find are the various forums and communities (particularly at Plex, Stex and Lex). But I'd like to mention this one site that has bundled all the basic tips (and some not so basic) on one single page. Useful to those that just need a quick refresher.