First is a group that I manage over on Facebook. The Tabletop Game Giveaways and Contests group boasts over 1500 members (and rapidly growing). Members are encouraged to share online board game (and related) giveaways that they find online. This is a great resource for discovering new games and increase your chances of winning some awesome prizes. I love seeing when members of the group win prizes (almost as much as I love winning prizes myself). So head on over to the group and join up!
Second is through promotions run right here on GJJ Games. Go to my Recent Giveaways, Contests, and Promotions page to find out what I'm currently running or have recently run. These contests may be for sponsored promotions from third parties, contests for copies of my own game designs, or even copies of games I've had the fortune to review (sometimes full copies, sometimes prototypes).
If you're interested in sponsoring a giveaway through GJJ Games I'd be happy to coordinate something with you! Simply contact me and let me know your idea. I recently organized a massive giveaway with 28 prizes from 23 sponsors that was a huge success. I'd love to help promote your game or product!