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.
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.