Bullet Bill 2
This game is Super Mario Brothers from the point of view of Bullet Bill 2. It is also very fast paced and challenging, especially on the higher levels.
The gameplay is pretty standard for a side-scroller. Use the arrow keys to navigate while trying to hit the breakable blocks, Goombas, and even the occasional Mario or Luigi. The challenge is to avoid the green platforms that you may remember jumping on from the original SMB games.
Analysis: Overall I think this is a pretty good game. Its graphics are reminiscent of Mario, the sound effects are quite good, and it is even fairly challenging. The later levels do get difficult, and you will likely need to memorize the entire level just to get through it. Anyway, like I said it is worth a try.
Jay adds: One of the nice things about this game is that when you die you only go back to the beginning of the current level, and you are given infinite lives with which to complete the game. And while the game comes off a little amateurish in production values, the simple and effective gameplay won me over. Play this one while you can, since Nintendo is generally quite adamant about protecting their intellectual property.
Update: And that was just a demo(!) The real Bullet Bill is out and the links above have been changed to point to the new, enhanced version. If you're still interested in playing the demo, you still can: