I like this addon, Boss Notes. Gives you an in-game notebook for each boss.
Comes with dot-point reminders of the key things for Ulduar bosses, for example, and you can enter your own. These can be broadcast to a channel you choose.
Another example of what a warlock might put in a personal note for a boss:
It also knows and learns the abilities that boss’ cast, so you build up a reference database of their spells, which are tooltips and linkable. I particularly like this feature, since sometimes this is all you need to check.
The addon Boss Notes has been under continual improvement for a while now, is regularly maintained, so appears to be a safe choice to rely on for your raid leadership notes.