There are a couple of tools that I use with Firefox for dev work and Safari 4 and Safari 5 have rather nice dev tools built-in. But I can see where other's would want to create a custom toolbar, a la Google, Yahoo, etc.
Do you use extensions in Firefox or Chrome? If so if which ones? And if they were offered for Safari 5 would you use them there as well? Would you even use Safari 5 if you aren't already? What do you think about Apple's announcement?
Learn more: http://developer.apple.com/programs/safari/