Sharebuilder has a $50 account opening bonus, if you open an account with them and do one purchase of a security. The disclaimer says that they will deposit the bonus amount in 4 weeks after you make your first purchase. You have to be a new account holder to qualify though. The offer does not apply to IRA or Educational Savings Accounts.
You can find the link here. Promotional code is SHARE50
Full Disclosure: I won't earn anything form that offer.