Ow.ly is great for shortening links and Twitter tools. PockeTwit is my favorite mobile client for Windows Mobile. Digsby is my desktop client, great for handling email, chat, Facebook, and all of your social media. Tumblr isn’t actually a Twitter client, but works great with your tumblog.
What other clients are “must have’s” for Twitter and blogging? Have you run into any security measures with any clients?