Or better yet, a simple userscript. That way a single script could be run on all modern browsers with a couple of small tweaks required for the big 3, and easily installable out of the box on Opera and Safari.
Indeed, many extension could be implemented as simple userscripts and as such could reach a much wider audience.
Not a bad idea in the geekier circles, but the idea of installing something that supports userscripts and then installing userscripts isn't an obvious step for many users, whereas Google pushes Chrome apps and extensions quite heavily as an easy thing to play with.