Spin the Black Circle
One characteristic these games seem to have in common is that they manage to improve upon their respective Nitrome counterparts, at least from the perspective of actually playing through the game. The same is true for Spin the Black Circle, by Alejandro Guillen, the latest game to join this list. It's hard not to compare Spin the Black Circle to Nitrome's Roly Poly (which in turn was inspired by one or more prior arcade games). Both require you to rotate a board while maintaining a constant gravity, and both present a variety of different obstacles, enemies, and surfaces. Yet, I think you'll agree that in most regards, Spin the Black Circle comes out ahead.
Not only is Spin the Black Circle smoother running, the physics also feel more natural and play a larger role in the level design. This could have something to do with newer and better versions of Flash released in the last few years. Inertia, all but ignored in Roly Poly, becomes the focal point for the challenges put forth. The first few levels let you practice the skills you will need later on in the game, such as navigating corners, pulling U-turns, and making floor switches. These can be tricky, because as you rotate the board, your inertial frame of reference rotates with it, but gravity always remains down. Expect to see your ball fly off into the spikes a number of times before mastering the controls. Then expect to see it fly off anyway as the levels get harder and harder. To succeed requires not just controlling your ball's inertia, but learning how to take advantage of it to squeeze through some tight spaces. To add even more challenge, Spin the Black Circle keeps track of your best times for each level, although simply finishing is achievement enough for some.
All in all, though Spin the Black Circle may not be nearly as cute as Roly Poly, but its superior performance, pleasing soundtrack, and more realistic physics do a better job of engaging the player, sucking them into that compulsive "just one more time" spiral of anti-productivity that we all love and hate to find ourselves in.
Cheers to Alejandro, Bob, L0ser, slgalt, Jacob, and Kristi for suggesting this one! =)