Stopwatch automatically appears when you log in and starts counting.

If you'd like it to count down from something such as 3 minutes, open Stopwatch.lua in Notepad or any text editor and edit line 4:

Change:

if event == "PLAYER_LOGIN" then Stopwatch_Toggle(); Stopwatch_Play();

To:

if event == "PLAYER_LOGIN" then Stopwatch_StartCountdown(0,3,0); Stopwatch_Play();

"...Countdown(0,3,0)" corresponds to hours, minutes, seconds.

This is potentially the most useful addon you will ever install.   I use this on bank toons and such to make sure I don't waste time in things like arguments on trade. If you're a casual player, it is also helpful if you're trying to cut down on the time you spend on WOW.