Saft
Saft is a plugin for Safari. Some days it seems the other way around. Safari is an Saft extension because Saft adds more useful features to Safari than the other way around.
I was wondering what feature I find the most useful, the most "I can't live without this". Its not the way it allows you to reorder tabs, and not even the type-ahed search (similar to the Firefox incremental search). It could be the session restoring features, that kick in to restore all your open tabs in case of a crash (giving you an opportunity to pick and choose which tabs to restore).
For me, the $12 feature (yes, it just costs $12...) its Cmd-Z. In those situations where you get Cmd-W-happy, and close that tab holding some important page you had visited, Cmd-Z will restore the tab just closed. And more: keep hitting Cmd-Z and all your closed tabs will come back to life.
That alone, makes it worth the registration fee.