I personally use only a couple of extras for Safari, as everything I really want in a browser is built in. I use
ForgetMeNot which is a session saver type extension so when I close a window, force quit or the like I can re-open to the previously viewed pages. I also use
Inquisitor, although I'm not so convinced of it's use, it's been there for a while now and I kind of forget it's not the default.
I also use a plist haxie which decreases page load delay. If anyone is interested I can post the terminal command, but I haven't seen a lot of that around this forum yet so I don't want to upset the status quo.
I am also looking for a way to re-organise my tabs in Safari, but don't want to install a big application with loads of features, just allows dragging of tabs. I know this is built into Safari in Leopard and so I will probably just wait for that.