Twitteron

Twitteron – Adobe AIR Twitter client (Windows, Mac, Linux). Features include changeable skins. Ooh.

Canary

Canary – open source Twitter client for Macs.

Syrinx

Syrinx – customisable Twitter client for Mac OS X. Lets you bookmark Tweets and search. Quite interesting and attractive looking, but initially a little confusing and not as straightforward as something like TweetDeck.

Chirp 2.0

Chirp 2.0 – Mac desktop widget for posting and reading Tweets.

Twitch

Twitch – Adobe Air Twitter client for Windows and Mac.

Twucket

Twucket – minimalistic Twitter client for Mac OS X Leopard.

Tweet = Twitter + Quicksilver

Tweet = Twitter + Quicksilver – tutorial from codahale.com on how to Tweet using Quicksilver (Mac).