Okami is a PS2 game from Clover, the same studio that did Viewtiful Joe. It came out in 2006 and was reviewed very highly, but didn't sell very well. More recently there's been a Wii port of the game. I've been playing the game off and on for the past six months or so, and finally finished it today.
The game's biggest draw is artistic. Its visual style is based on Japanese ink illustration and is beautiful from start to finish. It really looks like an animated watercolor painting, and it's pulled off very well. The music is also good. Dialog uses the artsy "voices speaking in gibberish" style used in a bunch of games such as The Sims and Ico.
The gameplay style is "action-adventure," meaning it's very reminiscent of Zelda games. You run around in a 3D world as a wolf, jumping and beating up monsters using weapons you wield from your back with some kind of mysterious wolf fu. Being a god, you also have the power to alter reality with a growing arsenal of "brush techniques," which you use by freezing time and drawing on the screen in specific ways. In the Wii port, you obviously use the Wii remote to draw. I played on the PS2, where you hold down a modifier and use the controller analog stick. The game isn't especially challenging but does make you think about what power to use on a fairly regular basis.
The storyline is fun and at times moving. I wouldn't say it's deep--evil god of darkness comes to wreak havoc on the mortal world, is opposed by plucky but temporarily weak wolf god--but it's varied and entertaining with interesting characters.
I spent a little under 60 hours on the game according to the final tally. I wasn't terribly completist about it.
Overall, I'd rate this higher than any Zelda game (better story, better art, better gameplay) though I've enjoyed games in other genres more. Definitely worth picking up if you have a PS2 or a Wii and some free time.