We reported earlier that Twitterific, from Ikon Labs was a solid choice for a Twitter client. (if you don't know what Twitter is, please see the Twitterific post). However, Twinkle from Tapulous easily outdoes it from both a UI and features perspective.
Twinkle's key differentiator is that it is location aware. By tapping into the iPhone GPS, it adds your current location to all of your tweets. But that's just not all. A "nearby" tab shows you all the twinkles near by. So for example, as I twittered that I was trying twinkle out, a fellow named Gary in Long Beach saw me and responded. A was able to immediatly follow him directly from the iphone -- something not available in Twitterific.
In future releases, Tapulous should allow twitter friends (but probably not everybody) to drill down and see the location with more granularity. I really love this app.