Safari 4 adds a plug-in isolation feature; this is similar t Google's Chrome in that when you launch a plug-in it opens that plug-in in its own space. This means that if the plug-in crashes you do not lose the whole browser just that tab. (IE 8 has this also)
If you are interested Safari 4 should be ready for live download later today. Just remember that Safari while blazingly fast is also one of the easiest browsers to hack.
Read more here.
The biggest news, though, is that, like Google Chrome, Safari 4 now hands each plugin off to its own process. So if you're watching a YouTube video and the Flash plugin crashes, only that tab will crash and not the whole browser. God that makes me happy.