I use www.flock.com to watch everything from Twitter, FB, Blooger, Aim, You Tube, ff etc. And I also use www.twhirl.org to moniter all tweets. Addictive though ;-)
I recently started using TweetDeck and I also use Twirl. Last time I checked, Digsby was only available for Windows, so I am waiting on the Mac version. I also like TweetGrid and Tweetchat for keeping with with the #gno parties. Another favorite of mine is Brightkit - http://brightkit.com/. I like to schedule inspirational quotes for people to see and I just like that app in general.
I'm using Flock as my browser, I use Tweetdeck and on my Iphone I use the Twitter App. If you're using Wordpress as your blogging platform, there are some brand new plug ins that I'm trying right now. Tweet back and the Tweet Suite from @danzarrella. http://is.gd/fwot
I use TwitterFox and TwitterBerry all the time. I dl'd TweetDeck, but it kind of gets forgotten. I can't wait until Google makes apps available on Chrome.
I have been using www.twitterfeed.com and really like it. It tweets rss feeds from my websites for me! So I just update the content of my pages, it makes an rss, and then it gets tweet'd!