Time to geek out, and I have just the sites to help you do it!

Mental Floss: Based on the magazine of the same name, this is a must-stop for your daily dose of trivia and challenges, to help satisfy your know-it-all side. Their Twitter page and Amazing Fact Generator are great for quick drive-by knowledge.

Sporcle: Fans of my How Many Xs Can You Name In Y Minutes? post will instantly recognize this choice. It's one thing to get trivia facts into your head, but how good are you at retaining them? These quizzes will challenge your ability on even the most bizarre and obscure topics!

ThinkGeek: OK, this is a pure commercial store site. It's filled with stuff you never knew existed, but that any geek will instantly find many things to lust after!

Plus Magazine: This is a free online magazine dedicated to helping people understand all the math around them. For everything from bell-ringing to traffic jams, it's amazing to discover just where math is being used to improve life.

XKCD: For newer geeks, this comic strip is a great place to start, because it challenges you to figure out what is so funny about the topic. Naturally, geeks who have been around longer pride themselves on getting the jokes without looking anything up.

Final Answers: Granted, the table of contents here looks scary, but click around at random, and see what you discover. The complex topics are often made fun and interesting, and you're guaranteed to learn something new, even about things you thought you already knew!

Wolfram Research: The main page here is dedicated to their flagship product, Mathematica, but begin exploring over in the OUR SITES menu. Among the great places to explore are Wolfram Alpha (a natural language search engine), MathWorld, a site dedicated to demonstrations of the math used in Numb3rs, and much, much more!

Television Tropes & Idioms: You're going to get lost in this site. Even XKCD can't escape the lure of TV Tropes! This is filled with those things you keep seeing in movies and TV shows again and again. Start anywhere, and start realizing how common many aspects of your favorite entertainment really are!

BrainyQuote: Here's a 2-in-1 site for you. Besides the main quotation site, featuring quotes on just about every topic you can conceive of, there's also a history site, as well. For example, if you look at a page of quotes by Jim Carrey, you can also click on his birthday, January 17th, or the year 1962, to see what historical events happened. It's not hard to get lost in these twin sites.

Lifehacker: Here's effective advice that can be put to use to improve just about every aspect of your life. Usually, the tips and tricks you learn can often be put into immediate use, as well. While software is a favorite topic, just about anything that is handy and geeky is sure to be found here. C'mon, what internet geek wouldn't love a Lego router?

Did I forget any of your favorite sites where you like to geek out? I'd like to hear about them in the comments!