We used Rafflecopter for out successful scrapbook software giveaway. It's a snazzy system that makes it easy for lots of people to participate and get the word out about the contest on your website.
a Rafflecopter giveaway
The winner is selected by a (pseudo) random number generator, but you can also look at the administration window for your contest to count the number of entries, for example, to pick a winner by the quantity and the point score of their entries over time.
They also have a contest to name their mascot, the smiling Rafflecopter.