"PartnerNet? Is that some sort of Xbox Live dating service?" A valid question, but one regrettably met with a negative answer. PartnerNet is a section of the Xbox Live network specifically reserved for and accessed by Xbox 360 debug units. As such, it contains unreleased and even unannounced downloadable content reserved for future migration to public servers.
Embedded after the break you'll find a PartnerNet video tour of sorts, as sent anonymously to CheapyD at CheapAssGamer (sans soothing narration).
Since the video was likely recorded quite a while ago, most of the listed games are familiar ones. Things to look out for are Rare's Jetpac, Juiced 2007, Absolute, Obbes Pinball, Tetris Online and Halo 2 (likely referring to the upcoming Xbox 360 multiplayer maps). Exciting, isn't it?
[Update: Microsoft has politely asked us to remove the video for several understandable reasons.]