I’ve been using the Saft plugin for Safari for a week now. I’ve barely scratched the surface of what it can do, but I’m finding it indispensable so far. At any one time, I typically have about four browser windows open with 10-15 tabs in each, so the auto-save and restore tabs feature is a life-saver and worth the $12 cost. Check out the site for more features.
Now, someone make something like this for Mail.app…just a few small features here and there would really make it a whole lot better.