Heli Attack 2
http://www.miniclip.com/games/heli-attack-2/en/
Although there is a Heli Attack 3 (and presumably a Heli Attack, but I have never found it) I have found that the second is overall more fun, although perhaps this is nostalgia on my part as the second has been around for a fairly long time. Heli Attack 2 and N were my Mac-friendly saviours in my high school information technology class.
The controls are similar to that of Abuse where the arrow keys control your movement like a regular platform game while you aim with the mouse cursor. A bullet-time slow motion is activated by holding Shift and is useful for getting a steady aim, while Control activates a super jump when you are on the ground. End changes weapon. The controls can be changed on the main menu.
The super jump seemed useless to me until I found out that if you kill a 'copter while you are in the air above it you get a small amount of ammo for a random weapon. This is an extremely important tactic as having a wide range of weapons at any time is crucial to survival.
One of the main things that struck me was the presentation - it looks and feels more like a good Game Maker game than a flash-made game. It is smooth, responsive and glitchless and has some good, if a little under-animated graphics that remind me of Metal Slug.
Overall it's a simple and fun game which can get a bit repetitive, but if you like platform shooter games and high scores, you'll probably like Heli Attack 2.
Heli Attack 3 may be worth a look at as well if you prefer more structure in your platform games - in my opinion the additions overcomplicate a winning formula but it's definitely worth a look at if you find the first (or second in this case) too repetitive.
http://www.miniclip.com/games/heli-attack-3/en/