Buy.com is having a big sale of Stargate DVDs, including both Stargate SG-1 and Stargate Atlantis. The prices are quite awesome, you can get SG-1 DVD sets for as low as $17 per season, and also for as low as $17 for Atlantis seasons. This is great if you are filling up your collection with seasons you don't already own.
But that's not all, they have plenty of other DVD and TV-on-DVD sales, including movies as low as $6 and things like that. Check out their TV on DVD sale!