Wizards posted up a dedicated memoriam to Dave Arneson on his recent passing. They retrace how Dave Arneson and Gary Gygax met at Gencon in 1969 and he came up with the idea of role-playing, using Gygax's Chainmail mechanics.
Dave Arneson was also responsible for the first campaign setting (Blackmoor) and the first adventure module (Temple of the Frog).
They're quite a bit of interesting information about Dave written here that some of us may not be aware at all. So, I highly suggest that you read it to find out more about this not so well known man behind the game and what were his contributions to it.
Finally, the memoriam ends with gratitude that says,
Whether you're a lifelong D&D player or a newcomer to RPGs, a traditional paper-and-pencil gamer or an online roleplayer, we all owe a great debt of thanks to Dave Arneson and his groundbreaking Blackmoor game.