Boss Notes is an addon that has been around for a while, and which I am surprised I have not had as a PotW in the past. This addon allows you to store custom notes on each boss you come across. It also comes with a pre-compiled list of tactics for some bosses. In addition, it records boss and trash abilities as well as boss emotes that it sees while you are in combat so that you can reference these when revising your tactics. The dungeon journal does not cover every instance/raid, so these last two features are still useful for other content. There is also a plugin for LibDataBroker (LDB) displays so that you can see your tactics/notes for the current boss. Perhaps the greatest feature, though, is that you can share your notes with your guild, party and raid.
Read more about what you can do with Boss Notes at its download page.
Are you the author of a mod that you think belongs in this spotlight? Just upload/update it to WoWInterface, and it will automatically be considered for that week's Pick.
"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff, The Tao of Pooh