Mailplane rocks. It's more of an SSB than a pure native app like Twitter for Mac, but it has a lot of nice little integration points with Mac OS X.
It's not cheap, but it's well worth the price (its also due for a paid upgrade soon, IMO, as I have not paid anything beyond the first version three years ago).
It's not cheap, but it's well worth the price (its also due for a paid upgrade soon, IMO, as I have not paid anything beyond the first version three years ago).