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There's only one Twitter, and there's only one iPhone OS, but for some reason there are more iPhone Twitter apps than you can shake a stick at. We took a look at the best free ones, and discovered that you can get quite a lot for your lack of money—as long as you don't mind an occasional banner ad or prompt to upgrade to a paid version.

Twitter may be simple—posting 140-character messages and reading them from your contacts isn't very involved—but you'll still need to ask a lot of questions to find the mobile app that's right you. Can it display hot trending topics? Does it let you follow and unfollow others? Does it have an integrated Web browser? Can it shorten your tweets? Does it let you easily post a Web address you saw in Safari? Does it allow multiple accounts and UI themes?

Some iPhone apps even let you do things you can't when tweeting from your PC. Most can display "nearby" tweets, even letting you decide how near or far their maximum distance should be. You can include your exact location with your tweets; some apps, such as the excellent Twitterrific, also let you include a map link.
These apps also let you tweet a photo taken on the spot with your iPhone, but none of the free ones lets you post video from your 3GS —currently only the exorbitant ($5) Twittelator Pro lets you do that. We'll be posting an in-depth review of that as well as of another paid app, the crowd-pleaser Tweetie, in the near future. But as you'll see in the reviews below, there's a free Twitter client that will please all but the most demanding tweeps.