Here is a topic that I thought I'd dedicate a thread to, since it will be buried in the LoL thread in no time. http://www.probuilds.net http://www.mobafire.com http://www.lolpro.com http://www.solomid.net http://www.championselect.net/ http://www.lolking.net/ http://www.teamsummon.com/ If you have any other sites, let me know.
Please don't use Mobafire :c Quality will be better on other sites such as lolpro to get an idea of which items. Don't just follow blindly either.
I like LoLPro's guides, since they care to explain what and why. Most of the build and champion guides on solomid are just put point there, buy stuff here, and that's it.
The majority of it is shit, what's the point of sifting though heaps of junk for some gold when you Can get it more easily on other sites?
The difference being that any single Mobafire guide can get to the top of the list if it gets enough votes. That creates a system where the most upvoted stays the most upvoted because newbies will upvote the top guide even further. Any random guide will work for bronze level play so as far as they're concerned it's all good stuff. Solomid and lolpro only feature and approve the highest level of guides, so you know you're usually getting something worth a try.
Added http://www.teamsummon.com/index.php It's a site where you can find solo/duo queue partners as well as maybe make your 5v5 team or find teams to scrim against. You verify your account easily, and they pull matchmaking information out. You say what lane you play, and what you are looking for. The server then finds people at your level.
Do not forget about https://www.lolvvv.com/ - to find the best League of Legends probuilds and runes or a specific champion build.