Adblock Plus - AdBlock Plus is a browser extension for Firefox, Chrome, Opera, and several other popular browsers that prevents intrusive ads like pop-ups and malicious code from appearing on websites you visit.
Tweetastic - Better Twitter analytics, scheduling and more
BlockParty - Real-time voice chat rooms from the creators of turntable.fm
ilo - Premium Twitter analytics
Twitter One Click Block - Add a block button for a simple one click blocking.
Accountanalysis - A tool to evaluate Twitter accounts