Firefox/Chrome/Internet Explorer: Tweetfilter is probably the most powerful filter you can find for your Twitter stream, unless you start decoding Twitter's API yourself. You can block phrases, retweets of certain users, particular web sites and services and nearly anything else.
Tweetfilter is a user script that installs in Firefox, Chrome and (as a bookmarklet) IE, and all it really does is create a box on your Twitter.com page where you can type in rules and settings. The filtering options are explained at Tweetfilter's userscripts.org page, and you'll see that beyond filtering of phrases, accounts and sites you don't care to check out, you can also edit the Twitter.com page itself, removing features, auto-showing new tweets and other nit-picks you might love to have in place.
The script only works on Twitter.com's "new" interface, but given that it's a fairly robust web app these days, that's not such a bad restriction, given your filtering abilities. It's a free download, with install instructions included at the link.