PSP Multiplayer UMD Sharing
Metal Gear Solid, Ridge Racer, Namco and Tony Hawk Underground are all confirmed to work with only one shared UMD.
The process requires two people we'll call Chris and Ponzi for demonstration purposes only. Chris launches the game in their PSP and activates an ad hoc wireless network. Chris opens UMD drive without quitting game and hands UMD to Ponzi. Ponzi loads game from UMD and joins the game Chris started. Both PSPs will stall temporarily at this point. Ponzi gives UMD back to Chris, choosing not to quit game. Chris puts UMD back in long enough to load game, removes UMD (again without quitting) and gives it back to Ponzi. If you survive this convoluted series of steps, 4 out of 5 marriage experts agree you're in for a long and happy life.
Update: This feature is made obvious by the orange oval with the words Game Sharing on the back of supported games per my non-scientific sample of games available at Best Buy.