Royal Vegas Casinos are offering 300 free spins of their online slot machines with no deposit required. Any money you make over the course of the spins is yours to keep. Move quickly however, as the number of free spins decreases by one every hour (currently at 295).
The terms and conditions don't mention anything about having to deposit to withdraw winnings so presumably this isn't an issue. The only catch seems to be that you have to download their software.
Should add that you need to get over 300 credits to be able to withdraw anything. Worth a punt at the very least.