I found this flash game a few months ago, and I find I keep coming back to it. I suppose that's how you can tell when a flash game is good or not.
It's like whack-a-mole with little green aliens. You use your keypad to punch the little guys with your PIKOPIKO HAMMER, and if you're lucky they leave behind little coins. Later in the game, you can use the coins to upgrade your hammer, buy extra lives, and so on.
The game has quite a few levels, I've only made it to level 8, and each level brings on a new type of alien to hit. I encourage you to check it out.
Like my blogsite, it's the perfect way to waste a day.