I've just uploaded a new release of Open With to AMO, with some major new features.
First, and probably more importantly, Open With now works on OS X and Linux. However, on those platforms all browsers need to be added manually, as I've no way of looking for installed browsers as I can with the Windows registry. Yet.
Second, and also important, you can now add browsers manually. This is a really handy feature which was requested a fair bit. Now you can add Open With menu items for browsers without them being listed in the registry. Which is especially useful if you don't have a registry, like if you're a Mac user.
If you use Open With, get to AMO and get the new version, it won't auto-update (see rant below). And drop me a comment saying you think it's cool.