is a powerful plug-in for FireFox that moves many disparate services and capabilities into one central location - right in your browser. This is almost a complete Twitter experience for the general and even power use , one that Twitter itself should provide.
Power Twitter is my favorite when I'm already browsing online, or checking my feeds in Google Reader, or otherwise just wanting to quickly tweet, reply or retweet. It has a clean simple interface and since Firefox is my preferred browser on my Mac, that happens rather often.
When I'm intending to spend some time connecting and catching up with others, I will open a desktop app like TweetDeck or my current favorite, HootSuite -- both of which have nice iPhone app versions too.