Nearly a month after Joystiq spotted it, we finally had a chance to sit down and play the excellent PSP version of Cave Story. The PSP version of this freeware game has the loving approval of Studio Pixel, the game's original creators, and is "based entirely on Pixel's original code."
The game is absolutely massive, offering just as much (if not more) gameplay as many retail games. We were really impressed by the fluidity of the port. A homebrew-enabled PSP will be required to play this incredibly well-crafted platformer/shooter -- check it out here.