With this timer, you choose a list of spells to monitor and those bars show up in fixed locations on the screen.

This makes it easier to maintain a set of DoTs on a mob. Rather than looking at a bunch of timer bars and trying to figure out what spell is missing, you can tell immediately what spells are on the mob and what are not.

To configure it, type /tb menu or just look in the blizzard interface menu.

Features:

Supports all classes
Supports both DoTs and beneficial effects
Separate displays for main and focus targets
Bars show number of seconds left rather than the percentage of the spell's total duration left. Max number of seconds shown is configurable.
You can configure when the display is up: Always, only when you have a valid target, only when you have effects on the target.
Some spells are handled specially:

Any warlock curse will be displayed in any curse timer. So only monitor one curse effect, what ever curse you cast will be tracked on that bar. Hunter stings and Paladin judgements work the same way.
Some effects will be displayed even if cast by someone else. Most of the CC spells fall into this category. Warrior's Demoralizing Shout and Thunder Clap also work this way.
This also works for other debuffs with the same effect. Monitoring Thunderclap, for example, will also show Frost Fever if it is up on the target.
Any suggestions as to other effects that should fall into these categories are welcome.