"Zombies have taken over the world and you are the only hope for mankind. Kill them all or die trying! Features lots of weapons, power-ups, NPC's, quests and the most important thing: lots of zombies to splatter across the screen!"
It's not exactly online, but you can download the demo here. It's still not complete, so this is just the beta version. I've only played through the first three levels so far, but I can already tell I like this game. It reminds me of old-school platformer games, back from the run and gun days; games like the Contra series and the Metal Slug series. The best part? It's all made by one guy (or girl), 'Blue66' being the pseudonym used. The game is hardcore, and by no means easily done. Even the creator states in the 'readme': "This game is made for mature audiences. If you're a sissy-man, please do not play this game. Thanks."