If you have a website that you would like to promote, there are several coupon codes out there that can get you a
free $200+ in credit towards Facebook Ads (update: codes are expired). In addition, if you join the Visa Business Network and install application, they’ll kick in another $100 (still available). Via this SlickDeals thread.
You’ll need a credit card to sign up, but it shouldn’t be charged until after you use up all your credits. Watch the expiration dates, and set your ad budgets wisely. Facebook ads are pretty low-converting from what I’ve heard, but at least this way you can test for free.