Rafflecopter is a pretty cool app that lets bloggers like me and even bigger companies run a giveaway or contest. The system is pretty simple. As a participant, you simply have to complete the different mini-tasks presented by the rafflecopter app and each task you complete gives you one or more entries towards the giveaway. At the end of the giveaway period, a winner is selected. Obviously, the more tasks you complete, the more chances you have to win. Some tasks can be done each day, so you end up racking up points.
I've ran two contests like this before. One of them is going on right now and I'm giving away three copies of Jeremy Schoemaker's book: The Shoemoney Story. Check it out here.