If you have five spare minutes, Void Ace will fill them. Everything happens on one screen with 3 lives, bosses with 65 base HP every third wave, and the whole thing runs straight in the browser - no downloads, no sign-ups, no fuss.
Every third wave ends with a boss that soaks real damage and answers back. Regular enemies come in patterns - sine sweeps, dive bombs, wall formations - and each pattern has a spot where it cannot hurt you; finding those spots is the actual game.
It suits anyone with a queue to wait in or a kettle to watch: students between classes, commuters, parents stealing ten quiet minutes. There is no meta-progression to lose and nothing to catch up on - a week away costs you nothing but a little sharpness.