Chrome: If you want to add some commentary to someone else's Tweet, Classic Retweet for Chrome builds the option for an old-style retweet into Twitter's web interface.
The extension adds a "Classic Retweet" button next to Twitter's standard retweet button in your timeline. Clicking it brings up a new tweet window with "RT," the user, and the tweet already filled in, allowing you to add your commentary or make edits. It's a simple concept, but it can save you a few seconds of copy and pasting. A lot of Twitter clients have this feature built in, but with Twitter shutting them down left and right, it's nice to have the option on the web interface.