Some of you guys may know the website xkcd.com, but if you don't check it out! It is a really funny webcomic about "romance, sarcasm, math, and language," as they bill themselves.

Here are some number theory related comics I found:

Prime numbers:

http://xkcd.com/5/

http://xkcd.com/247/

http://xkcd.com/521/ [this one is slightly less math related, but it does talk about primes at the top!]

Sigma Function:

http://xkcd.com/410/

Math teachers:

http://xkcd.com/263/

Mathematicians:

http://xkcd.com/435/ [this will just make you feel good about being a math major!]

What a fantastic website.