Forget Me Not 3.0 is a good example of the 3rd party plug-in and add-on support for Safari that is slowly but surely growing. FMN is a basic a session saver plug-in for Safari that can reload any tabs that were open the last time you quit Safari. It isn't quite as powerful as, say, Tab Mix Plus for Firefox, but it does a good job of bringing this increasingly popular feature to Apple's darling browser.
Forget Me Not can be had from Jim Fowler's site.
[via Daring Fireball]