The underlying rendering code in Safari is based on KHTML which is open source. It turns out that new builds are available here and what you get is a version of Safari that has new features. The app is called WebKit and it can co-exist on your Tiger system along side current Safari. It picks up all of your settings and bookmarks so it's an easy transition.
They seem to build very frequently, often several times a day. I'm posting this with the Friday 23rd Feb build and it seems solid so far.
You get a nice gold rimmed icon too.