Gungame is an awesome shooting io game where you start with a small selection of weapons, but continue to grow your arsenal as you progress.
To play, control your character with the WASD keys, and shoot at other players with left click. You can also scroll to switch weapons.
A really good gungame strategy is to try to get the other players to follow you, and shoot backwards. This is powerful because when you are moving backwards, it makes it really difficult for their bullets to hit you, and you can quite easily hit them with your own.