Is there a toolbar for Stack Overflow that can be added to a browser window?
There is in fact a Stack Overflow toolbar for FireFox, Stack Overflow Add-on
It has fairly limited functionality at the moment, but it's pretty hard to do much without an API (unless you use GreaseMonkey as already pointed out, although still less than ideal).
The Powers That Be could get a free one from Conduit and then also make a few cents everytime someone installs it.
No adware, no spyware. I use it on one of my own sites and people are happy.