Probably a giftcard or some cash. It's hard to decide what to give as not all users will be interested in the same topics on the forum, some people may want a video game, others may want tickets to see a musician. It's hard to choose so I would suggest something like a giftcard for a store that sells lots of different things, so depending on what country you're in, you could choose Tesco (UK) or Wal-Mart (USA) or a similar store.
A giftcard is generally a great way to go, but let them choose which company they want it from. This gives them the benefit of actually trying to win something more valuable than software etc. Books, DVDs are also a great hit although giving away an electronic item is good too. Many people would join the contest for lets say and eBook reader such as the Amazon Kindle.
Everybody can't use gift card/paypal cash if you are focusing on all visitors(global). Because everybody foesn't shop online and paypal is restricted in different countries. I would suggest to put free domain name/free hosting as optional prize if they don't want $.
It depends what sort of crowd you would like to bring in. You could target two different types with two different prizes. To get gamers in you can use Games, giftcards, Microsoft Points even game consoles. Or if you want older more mature people - you could do two tickets to any local spa or a weekend break. Depends on your budget. Think outside the box. If you make your competition about something you could market the competition on another site for free as long as it's inline with the genre.