SurfRabbit is a pretty slick browser add-on for Safari, NetNewsWire and Shiira (all webKit) which works in a similar way to Greasemonkey does. You can create and share hacks to sites that enable them to work better for you. I’ve just installed it and find that it seems to work as expected without issue.
I really like being able to clean up pages like News.com full articles which I tend to read many times a day. There are about 20 page tweaks currently in the system, though I suspect many more will follow.
I’ve been using it to clean up Fark.com and Atom Films. Still can’t remove their intro movie ads though ;^)