It's a mixture of vBulletin, Editize and custom (in-house?) programming / integration. I don't know of any other 3rd party applications used for SP other than vBulletin and Editize. The SP Marketplace & Contests section I believe are custom and integrated into the vBulletin forum software. My guess is that the other applications integrate into the vB user database and may or may not use their own databases for app specific content. Probably just the vB database for everything. I don't recall SP ever disclosing exactly how they developed the site, so everything beyond vB & Editize is just speculation on my part.