I love the MLB '07
demo (I play it over and over, waiting for the PS3 version) and the conquest mode in NBA '07
is reason enough for me to buy it when I see it for under $20.
The Loco Roco
demos, of course, are fantastic.Killzone: Liberation
is definitely worth the download if you haven't played it.
I'd recommend the DJ Max Portable 2
demo, as well.
Come on people, feel the PSP loooooooooooove! (play NBA '07 Conquest mode!)