Engadget just reported about HTML5 gaming as well which reminded me that I had tried one such site awhile back. I have no knowledge of any examples working on pre-2.1 devices and only semi-working on 2.1 (Nexus One is what I tried it on). Engadget just posted some more examples you can certainly try out (one of which includes Akihabara). Unfortunately until we have full HTML5 support within Android, most of these examples will only partially work (if at all).
Engadget doesn't mention trying with Android and as far as I can tell only Akihabara decently works. They have a few games up right now you can try out from a Tetris Clone (hah!) and Pac-man to an 8-bit RPG game!
These are right from Engadget and I haven't tried all of them as of yet but I do know if you head over to Akihabara in your Android browser, you'll be able to test out the demos that are available to some degree. The future of Android gaming is pretty bright now with Flash 10.1 support and soon HTML5.
I think until HTML5 is more polished that Flash will remain (and probably will remain) the leader with browser based gaming, especially on Android but at least we'll have more options in the future. If you get any other examples working, let me know in the comments including the version of Android you used!
AndrewH - Andrew is the Founder and Editor-in-Chief of DroidGamers. Having been a hardcore PC and console gamer all his life, this passion is now applied to Android gaming and writing about it. When not playing or writing about Android games, he spends his time with his wife and son.