I hadn't heard of this one before, but I stumbled across Deepest Sender while poking through the Firefox extensons database. It's a XUL app, so should run nice and fast. At first blush, it looks like a pretty handy way to quickly post stuff. It took maybe 10 seconds to configure to point at my WordPress blog here.
The editing interface only appears to let you select one category, though, but nicely handles keyboard input to select a category quickly.
If you can read this, it worked. And you can even edit posts via the Post History interface. Cool!
I was initially looking for the Tab Mix Plus extension (which rocks, btw. get it.) Thanks to Chris for the tips!