A reader wrote in a few days ago with the question "What add ons would you consider helpful/essential for a new raid leader?", and as a raid leader myself, I thought it was such a good question that I'm devoting this Addon Spotlight to it.
Deadly Boss Mods

Verify any in-game mail you get that contain external links by checking your guild roster. Also, you should have an in-guild policy that limits external links to the member restricted area of your guild forums only. In all cases, don't enter your account information unless you go to the Blizzard website directly and scan your computer daily/nightly using any of the freely available virus/malware scans.

Boss mods are very helpful for all raiders, of course, but particularly crucial for raid leaders, who need to call out when raiders need to do things. If you're not familiar with boss mods, they basically aim to tell you any information about boss abilities that you might need to know during a fight. For instance, on XT-002, boss mods will tell you if anyone has a bomb debuff, when XT is about to throw a Tantrum, how long the heart has left, and so on.