I’m sorry to do this to you, but I recently came across not one but two long lists of good small games, any one of which you can start playing in seconds, any one of which you can blow thirty minutes on without breaking a sweat. Beware!
The other list is from a place called Cognitive Labs, which appears to be associated with Stanford: Free Cognitive Games from Cognitive Labs. It’s a weird, chaotic site dedicated to slowing down or reversing the cognitive effects of aging. The premise is that these games are good for your brain. I can testify that they are bad for your ability to go to bed at a reasonable time. Try out Vector Ball and Reverse Asteroids. Also I am specifically curious how long JMike can stay alive at the Impossible level on the type-fast-or-die Word Shoot game. If you’ve never seen JMike type, it is a wonder to behold.