Flow

Wow! This game is a demonstration of Jenova Chen's Master's thesis topic, "Flow in Games" - the short version is that games should adapt their difficulty to the player so that they stay in the "zone" where players are engaged but not frustrated. Whether or not that purpose was achieved (to me it seems pretty good), this game is lots of fun, and very aesthetically pleasing. Things get darker as you go deeper, and the enemies get harder - basically, your mission is to eat or be eaten. The red "pings" lead you deeper, and that's about all you need to know.

If you enjoy Flow, you'll also want to check out a downloadable game that Chen made, called Cloud:
I don't usually suggest non-browser games, but this one is incredibly interesting and well worth the download time. I hope and expect to see many more great games from Chen in the future. From his bio page:

One of Jenova's greatest dreams is to bring the best things and messages to the world. Like what Hayao Miyazaki did to anime, Jenova wants to be one of the heroes who can make video game a more mature and adorable treasure shared by every human beings.

I would say he's well on his way - these offerings are among the most interesting games that I've seen in a while, right up there in my book with Katamari Damacy (although on a less grand scale - but hey, they're free games, I'm sure if you put hundreds of thousand dollars into the guy's war chest you'd end up with something even more ambitious and compelling). Enjoy!