Super Crate Box.
The developer has set up a number of tiers for unlocking free content in the game, the first being five million. Players quickly surpassed that and reached ten million, prompting this 1.1 update today which adds new control options (jump and shoot!) based on player feedback, and two new unlockable characters. One is unlocked after collecting 1000 crates, the other by reaching 5000 kills.
Finally, Vlambeer teases a new update when 40 million crates have been amassed. You'll also save a princess or something, but we're sure you've already done that once or twice in your lifetime.