Suppose for a moment that you could go back a decade in time.  The original Playstation is achieving popularity, and the Nintendo 64 is still new.  The Xbox and its successor won't be available for years.  If you were to walk into an arcade, you wouldn't see many DDR machines yet, but there would be dozens of fighters.  Mortal Kombat, Street Fighter, and Killer Instinct ruled the day.  One on one fighting was by far the most popular genre, but there were a few notable exceptions that featured co-opgameplay.  One of those, a fondly remembered classic, one that spawned several sequels, was Metal Slug.

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All the Metal Slug games are great! Just pointless shooting up baddies...it always reminded me of the original Contra.

Great games. I believe Metal Slug 7 is coming to XBLA soon with co-op, something DS version lacked.

Yeah Metal Slug 3 is on XBLA. Check it out if you haven't. I only played the demo.