I was speaking about SIMBL just few posts ago. SIMBL is an extension able to add to the Apple's browser some plug-in written by third party author just in a snap. The number of these plug-in – unfortunately – is pretty limited, but imagine what's happened when on the last reboot – after having installed Snow Leopard on my MacBook Pro – I didn't see any of those I installed running again: an hearth attack.
Fortunately for everybody who started enjoy SIMBL, now there is a solution. The author finally updated it to run also on the new 64 bit operative system.
What is SIMBL? Originally it has been developed to personalize and block the Ad on the Apple browser, but it's powerful let it easy to use from third party programmer to create nice extension for Apple's browser.
Nothing has changed about the installation process: plug-in should be placed in the /Library/Application Support/SIMBL/Plugins folder, but remember to switch the run mode of Safari to 32 bit, at least until every single author of the plug-in won't update to the 64 bit version.