There used to be MobNotes but it hasn't had any notes preloaded for WotLK. It does give the ability to add your own, though. Not quite the same but there is also Ghost: Recon which will add mob abilities to your tooltips as you encounter them. Doesn't help the first time you meet the mob, but next time it can help you remember what the mob can do. It will also include information like whether or not the mob can be cc'd, but once again, you need to have encountered the mob at least once before so it can "learn".