Wikipedia is a top resource for reference and a hub of knowledge most people use it for reference. I also use it for same. After Wikipedia, many similar sites come in this field to defeat it but no one could be succeeded even Google could not. But there are some other useful sites that are similar to Wikipedia and have their own importance. I visit sometimes those sites also.

(1) About.com
About is an online guide on any social topic, there are experts to help you. You also can become a guide.

(2) Wikihow.com
Wikihow is a wiki having more than 76,000 on How To. There you can learn and contribute.

(3) Wikia.com
You can create your own Wiki here. Here you can find many articles on who is, how to, and theory etc. These are a questions and answers section.

(4) Answers.com
Here you can ask questions, reply answers.

(5) Reference.com
Refference.com includes dictionary.com and thesaurus.com, so here you can find references and search meanings in dictionary and thesaurus.

(6) Howstuffworks.com
Here you can find how stuff works, mostly here you will find technology related stuff.

(7) eHow.com
here you will find how to do just everything. You will find how to of the day, top how and how to guides.