Wowwiki generally has good boss strategies, wowhead.com also has some nice write ups. I generally like wowhead because you can also read people's comments about the bossfight. A lot of comments are bad but quite a few also have nice tips or good tactical info about the fights (look for the posts in green text).
There's also a lot of information on the forum here, both various kill videos and threads with info on different phases and strats and so on. If you just want to get an overview of common problems, try searching on "help" and the boss name or "stuck on" with the boss name.