ClickToFlash - Technically, this isn't a Safari 5 extension yet, but an older plug-in. I don't care. It just stops Flash from starting automatically. This is much less necessary with AdBlock and YouTube5, but it still helps contain the Flash damage.
YouTube5 - I joined the YouTube HTML5 video beta early. but it didn't do any good for most commercial videos or embedded videos. Then I discovered this extension. You have to quit the YouTube HTML5 beta, but you get HTML5 videos everywhere, even on embedded videos out in the wild. This extension and ClickToFlash will save my CPU from burning itself up.
autocomplete - A lot of places don't want you to autocomplete their passwords. On my Mac, I have my keychain secured with a secure passphrase. As best I can tell, the keychain is reasonably secure with it, so I would rather keep my passwords there. This one lets me do that.
adblock - It does. Practically no ads slip through any more.
There's some other extensions I'm still deciding if they rank as "must-have" or not. I'll let you know that later.