There are few add-ons for Safari if you look on Google... (like "Pimpmysafari" for example).
As far as I know, the "best" way to Tweet FROM Safari is to use BigTweet, a button to drag into your bookmarks; it will open a window, pop-up in the middle of your screen and allow you to post into your Twitter account. Don't forget to put "always" into the Safari preferences for your cookies and it will be just fine.