I have often made less than complimentary comments about CTAN: The Comprehensive TeX Archive Network, because I dislike its cluttered, ill-documented interface that in my opinion makes it needlessly difficult to find things (or to interpret what you have found), but when you get right down to it this is the only TeX/LaTeX site you really do need to have bookmarked.
Whenever I want to know if there exists a LaTeX package that does something I need done, I usually go straight to the CTAN Search Page, and do a keyword search through the TeX Catalogue (which also has its own website). If I cannot find it there, I can be 99% certain that it simply does not exist anywhere else either.
As a general rule, whenever you encounter limits on the functionality of the basic LaTeX packages and macros, don't underestimate the amount of work other people will already have done in order to extend those limits. Before you settle for something less than you wanted, or turn to programming things yourself, make sure that nobody else has already written a package that will do (almost) exactly what you need.
Although I have often found LaTeX help and information on the net, this has mostly been through Google and Google Groups queries. There are not that many specific LaTeX-related websites I have used regularly (apart from CTAN), but the TeX Frequently Asked Questions site maintained by the UK TeX Users Group is an exception that is definitely worth a bookmark.