First non-D&D you played.
The first game that wasn't "D&D" exactly was Castles and Crusades, but the similarities in game play makes me not want to count this one.
For me I would consider my first to be Victoriana 2nd edition. To put the reason down sounds silly but here it goes.
My wife and I both play Dungeons and Dragon Online, and Star Wars Online.We have been playing DDO for a while now actually. When I first started (before my wife joined me, but just barely before) I played alone for a while and when I mentioned I played to one of my friends, one of his other friends played also. He gave me his characters information so that he could invite me into the guild he was in on there.
Looking at it now I consider this a wonderful thing. The guild is called Blades of Serenity and the leaders of the guild are a married couple. To be honest the only reason we still play is to chat with them and a few other members (Mr. Anthony especially, who really helped us get started in understanding the game). Well to shorten this story now; My wife and I got the opportunity to meet the guild leaders and friends (Joe, Amy, and Anthony). When we did they asked about a tabletop game to play and I told them we were up for something we haven't played before, so Anthony chose to use Victoriana. All in all it was probably one of the best tabletop gaming experiences I've ever had personally, and we've gotten to meet up with them again (we played Deadlands that time which was awesome!)
We're planning on visiting them again so we'll have to see what else we come up with to play, either way should be fun.
I realize also that I got a little off subject with this post, but I still answered the question at least.