I have quite a few plugins that I use here at Literal Barrage, some of which have, at times, had a tendency to misbehave and make the site virtually unreachable. Without direct access to the mysql database underlying the site, it’s very difficult to correct these issues and, even with said access, it’s a pain to disable any wayward plugins or to otherwise muck about with the database’s contents.
Thus it is that I heartily endorse WP-Medic for any and all serious WordPress users. It has saved my bacon more times than I can count and, while not a plugin in and of itself and still in its “Release Candidate” phase, it has turned out to be one of the more useful and informative tools that I’ve used. Check it out if you’re using WP. I can’t recommend it enough.