Firefox with Greasemonkey: Reader Matt compiled a set of user scripts that make using the Twitter web interface better—adding a Retweet button, tab-completion of friend names, and more.
Once you've installed the user script, you can move your mouse over a tweet to find the new retweet button, scroll down to preload the next page of tweets, start typing your friend's names in the reply box and use the Tab key to auto-complete them, and use an inline reply form on the single tweet page.
This compilation was inspired by our very own Lifehacker Code extensions (Better Gmail, Better GReader, Better YouTube, and Better GCal), and while this isn't an official Lifehacker-sponsored compilation, it's great to see our readers creating useful new features and releasing them for everybody. Better Twitter is a free download, requires Firefox with the Greasemonkey extension.
Better Twitter for Greasemonkey [userscripts.org]