Baking23- buying individual stocks with $100 might be a fun way to get your feet wet and motivate yourself to learn more about the market. I'm doing something similar myself.
However, I consider this more of an entertainment vehicle than an investment vehicle for the reasons already outlined here.
I actuallly just wrote a piece in my blog about my first expereince with Sharebuilder. If you start with Sharebuilder make sure to use a coupon code, they are constantly being offered. eBates.com offers Sharebuilder deals from time to time, too.
My blog entry is here: <A href="http://threebeansalad.savingadvice.com/2007/02/16/lucky-me-my-20-investment-is-now-7743_22369/"> $20 investment is now $77.43 </A>