I'm glad you asked. Well, it's a very nice way to share fun Mathematica projects with friends, like you guys, like this one:
If the link is broken, I'm doing a bad job as QA. The page should be something like this, but interactive.
Fun, right? There are real numbers and equations behind the pretty picture. Some things that may happen in the future include telling you the values that you selected, or other relevant measurements (saddle height, set back, reach, drop, etc.). Eventually, I'll set up an API function which will allow users to specify bike size and body dimensions. Then, I will be rich and famous and powerful.
That's basically what I spend most of my days doing: writing fun stuff to make sure that our products work just fine. It's not a bad job.
Update: Joel reminded me that I can directly embed here. Voila!