Tweetdeck has completely changed the way I use Twitter.
In addition to the usual overview of people I'm following with options to reply to or directly message them, I can also choose to have several concurrent search terms locally and globally which show results from the last 48 hours and new results as they are tweeted.
Bryan had the nice idea of using it as an instant consensus machine for movies.
It also has a 'Twitscoop' function which trends all current activity on twitter. As you can see below, msn messenger's servers have gone down.
Makes me feel like Ozymandias.