Must-have mini titles for the PSP

What are your must have titles when it comes to the PSP minis? I'm looking to expand and I'm sure I probably overlooked some games. Zenonia getting the update it needed put it to the forefront of games to sink some time into. I really dig Fieldrunners, although I do prefer it on touch devices.

Any recommendations? I'm not looking to buy any UMDs or anything, so I tend to stick to downloadable PSP games and minis.