KLHThreatMeter (more commonly known as KTM) is definitely one of the most required addons out there for raiding guilds. With the addon installed on every member of the raid, the threatmeter can give a heads up to both tanks and DPS when aggro is getting unbalanced. But KTM is an older addon, and as a result, it's gotten a little shaky-- it's a bit of a resource hog, and some readings from it (especially when not everyone has it installed) can be a little off.
Enter Omen, a relatively new addon (part of the Ace set) that the hype says is a KTM-killer. The author says it's fast and clean and it seems much more customizable than KTM-- you can even pull out specific threat bars to watch. It'll also hook up to KTM, so if everyone else in your raid is using KTM, Omen will still work for you-- and probably better. One big drawback is that it won't distinguish threat between same-named mobs, but KTM doesn't do that either (it's a failure of the addon API), and there's not too many situations where you'd need something like that.
Lots of guilds have already switched over to Omen, and apparently more all the time. Have you used it yet? Does it match up to the hype? I haven't had a chance to get it running in a raid, but the next time my guild heads into Karazhan, I'll be watching to see how it works. If KTM hasn't been running as quickly as you've wanted lately (and you're looking for something harder, faster, better, and stronger), Omen might be worth checking out as a new way to handle threat.