Open a ShareBuilder from ING Direct and get $50 Cash Bonus.  There doesn’t appear to have any trading requirement.

You must deposit $50 or more to receive the $50 bonus. Please note the $50 credit will post to your account approximately 4-6 weeks after you have deposited $50 or more. Cash offer available for Individual, Joint and Custodial accounts only. The $50 offer is not valid with IRA or Education Savings Accounts and is not available for withdrawal for 90 days.