Abobo's Big Adventure is a crackpot homage to the NES, featuring Abobo, a boss from Double Dragon. It's a pretty vast game, for all its 8-bitness, given that it's Flash freeware.
Over the course of many levels, Abobo deals with a huge number of enemies -- seemingly every damn character that ever appeared in a NES game, though I'm sure that's not the case. The controls are NES-simple -- arrows to move, attacks with A and S -- but the gameplay changes drastically from level to level. At first, you think it's basically a Streetfighter-style game -- but the next level is more like an underwater Mario level.