This site has a few high quality browser games. The one I come back to is X Type, a bullet hell shoot-em up that has ever expanding enemy ship sizes, and never ends. It gets hard fast.
I also like Xibalba, which is a Doom/Wolfenstein style game playable in the browser.
The creator also did a rewrite of quake in 13 kb of javascript
Master Archer
Addictive arcade game about archery. Reminds me of flappy bird, not in the raw mechanics, but in the way they are both addicting in the same manner.