R&C:SM recently dropped to $6.99 used, but it's hard to find complete copies at local stores. Most just have the UMD that was included in the PSP-3000 bundle, which came in a cardboard sleeve thingy. I'm ordering it from the site once used copies come back in stock.
Definitely would be interested in picking up Guitar Hero 5, or World Tour for that matter. I've always been one to purchase the Guitar hero and rock band games used as the songs are usually classics anyway, so it doesn't matter when you get them.
Oh to find a used copy of Radiant Silvergun for less than a week's wages.
Other than that, Batman: AA looked like fun, but not quite enough for a full price, certainly a possibility down the line when it's cheap in the used game market.