Game Timer App (not to be confused with the similarly dubbed Game Timer) is exactly what the name says: a simple timer intended for use in board games and other activities. But I tend to use it whenever and wherever I need a fast, no-fuss single timer.
Game Timer App (free)
It has a clean, simple interface, keeps the phone awake while in use and offers increments of minutes and seconds. For a free app, I also appreciate the fact that it has no ads.
What you should know:
The maximum amount of time you can set is 59 minutes, 59 seconds and the alarm only dings once. The app also defaults to 1 minute whenever you open the app. Though this tracks time when the phone's sleeping, it does not have the continuity to account for lost time when you switch to another app or take a call. When you come back, the timer will be off and set to the default 1 minute again. (Be warned: If the phone rings and you're timing something important, you'll want to decline the call.)
There's a lot that this timer doesn't do, but it was never intended to do everything. It's just a clean, simple app that keeps a basic countdown - no more, no less. So for that, and the $0 price tag and ease of use, it's a winner for me when I need a simple timer for anything, except for maybe boiling an egg.