Another game that even offers you achievements is currently available for free in the store.
Aimlabs is an aiming trainer that you can download for free. If you want to improve your aiming skills, which we all should, you can use Aimlabs to fire a few shots and train your reaction time.
The game also has achievements and offers you 1100 Gamerscore.
Many thanks to @Rotten for the tip via our contact form.