Stockpile is a [relatively] new site similar to Sharebuilder. Right now, on Groupon, you can get $20 worth of stock for $10. You can even buy Bitcoin on Stockpile.
EDIT: Apparently, it has now sold out. It is still listed on the site, but I was just talking with a representative trying to buy one for my wife and I had issues actually buying it and that was their response.