It looks silly, but it's really a lot of fun and can be very challenging. You play the Loco Rocos, the little orange blobs. Apparently while you were napping, some alien flowers came to your planet and ate up all your little buddies. So you have to go around freeing them (and eating flying strawberries in the meantime). You move by tilting the world around you and rolling from place to place. Join all your little buddies together to form a big blob or split up to fit in tight spaces. Good, clean silly fun with great cartoon animations and very smooth gameplay.
Download for the PSP in English - http://www.psp-vault.com/UpDownload-req-viewdownloaddetails-lid-1108.psp
Play the Flash game on your PC - http://locoroco.com/index.html?locale=en_GB
Check out the official site (all in Japanese) to get a glimpse at the animation - http://www.jp.playstation.com/scej/title/locoroco/
The big news about Loco Roco (and the reason you see it all over the web recently) is that it is the first demo playable on the PSP without a disk. Sony just released a new firmware version allowing this.