I don't know about this site, but string theory is a serious and quite interesting scientific theory. It's really just an attempt at unifying the Standard Model (quantum mechanics and electromagnetism) with Relativity. This unification has become a very important goal in theoretical physics in recent years. So this business about extra dimensions isn't really the main point of string theory. What happened was that many years ago physicists discovered that uniting different forces becomes a lot easier when you add more dimensions (The flatlanders analogy explains how this happens fairly well (Flatland)). So the exrta dimensions weren't made up just for the hell of it, they're actually a requirement of string theory, and other theories before it.
Finally, I should point out that String Theory is, so far, just speculation. What is important about it is that it appears to be self consistent, something other unification theories before it weren't able to achieve. Now, the next step in string research is finding experimental corroboration for the theory. If that happens, it will be one of the greatest discoveries in physics of all time.
Other theories:
Kaluza-Klein Theory. (the beginning)
GUTs (Grand Unification Thories)
Supergravity
Have fun! :D