Many casinos now have daily freeroll slots tournaments. These are free to enter generally but some may require that you have been a recent active depositing player at the casino.
You will get a set amount of chips to start with and a time limit to play. Any wins will contribute to your leaderboard score. At the end of the tournament the prizes (which can be free cash or free spins) will be credited to the respective winners.
Sometimes you can rebuy or buy more chips to help you get further up the leaderboard with the chance of winning higher prizes. Some freeroll tournaments allow this, and some do not (freezeout type tournaments where everyone competes knowing that they all have the same chips to play with).
You can find a list of the latest Daily Freeroll Slots Tourneys here. These are updated from our friends at Casino Crush Forums.