It's been out for a while but I've only just discovered Leopard WebKit, a modern build of the WebKit open source rendering engine for Leopard. Safari is built upon WebKit but Apple stopped updating Safari for OS X Leopard mid 2011 and with the web being what it is you'd be mad to browse using that now as many security vulnerabilities have been discovered meantime.
Well now with Leopard WebKit you can keep using Safari but with all the current security patches applied. What the devs have done is take the last PowerPC build of WebKit and selectively applied the patches written since to bring the browser up-to-date security (and feature) wise. It doesn't actually replace Safari, rather when you launch it it starts Safari but uses Leopard WebKit as the rendering engine.
I've decided to change the OS X section to an Apple section, basically due to the fact that half the time I go on about wanting some Apple hardware and not really talking about OS X.