If you're Twitter happy and use Firefox....

Quoted: Twitterbar brings Twitter to your Firefox toolbar. Released under the GPL, Twitterbar adds a tiny icon to the right hand side of the addressbar. Type your twitter into the addressbar, mouseover the icon to see how many characters you have left and click to post; It's just that simple.

Twitterbar also supports URL only tweets, simply click the icon without changing the addressbar text, the resulting tweet will automatically prepend "Currently Browsing: " to the URL.