Solidarity Solitaire is a pretty good version of Solitaire on the App Store, and developer Kyle Davis just recently released a free version supported by in-game ads. Just like the paid version, the free version is a no-frills affair. Solitaire is Solitaire, though, so if all you want to do is play a little solo card game, it'll do ya. The two quibbles I had with the app were that the free version doesn't offer a landscape orientation, so if you want that, you'll have to pay the 99 cents.
Also, there's no auto-clearing of the board, even when you have opened up all of the cards. Update: The author of the app points out that there is an option for this. Then again, half of the point of Solitaire is just moving cards around, so maybe that's part of the game.
If you play a ton of Solitaire and want a full-fledged experience, you'd probably be better served by shelling out for something like Solitaire City. It's much pricier, but with multiple modes, better graphics, and even leaderboards (wait -- leaderboards on a solo game? Solidarity offers that as well, thanks to OpenFeint integration), it might be worth the investment if you play cards on your iPhone a lot. If you just want a quick hit of pushing cards around, Solidarity (especially the free version) is worth a download.