i cant believe noone mentionned Zombie Smasher X!!! this is a great side scrolling pure arcade/action game.
http://www.totallyscrewed.net/newsite/home.htm
http://www.rocketdownload.com/program.php?id=4077
you get to upgrade your character with weapons and special moves, theres so much to do its great!
http://www.gametunnel.com/articles.php?id=403 --game of the year on this site. haha