In Cavern Flier, you must fly as high as you can. Use cursors or WASD to control your ship, and space to fire. You can use your weapons to erode the scenery if it blocks your way.
From a personal perspective, I'm quite proud of this one. It's not quite so trivial to deform the landscape in flash as in other languages. I also like some of the explosion effects. It's a nice simple game, in retro style - it works, and I find it quite fun.
However, there's other issues... games like this in the past were on much smaller screens, so the issue of spare pixels wasn't so bad - you just shoot it out of the way. So my favourite feature here is that your ship won't crash into small groups of pixels - you can leave a bit of debris :)