If you like action that rewards reflexes and accuracy, Space War Symphony keeps every round tense and active. Movement and timing matter just as much as attacking, so success usually comes from staying alert instead of charging in blindly.
It also works nicely on quick breaks thanks to short rounds, fast restarts, and a clear objective from the start. To get started, click with the mouse when timing matters. If you want a free shooting game online with fast rounds and responsive action, Space War Symphony is an easy one to try.