Never as famous as Gary Gygax, we may owe more to the former than the latter. As I understand it, Gygax was responsible for adding fantasy elements to his medieval combat game, but Arneson was the one who contributed these:
1) "Hey, what if we each play just one character, instead of a whole army?"
2) "Let's make it open-ended, so the game keeps going and you don't just stop and declare whoever wins the fight the winner."
Both of which define roleplaying games for me far more than guys in plate armor ducking fireballs do.
Gygax got most of the credit, and it took a court order in 1981 for Arneson to be credited thereafter as co-creator of Dungeons & Dragons. The settlement bound either man from speaking of the details, and now they're both dead; we will probably never know the full story of who came up with what, when, and with whose help, why they fell out, etc.. I don't wish to disparage Gygax's name -- he seems to have been the driving force behind D&D's original publication and its spread, if not its inception. We owe him a lot, and he was by most accounts I've read a great human being. Business, or so I gather, can be hard on friendships between even quite reasonable people.
And, I mean, I never knew either of these people personally, nor even met them. I can only remember them in a very abstract manner. But I do feel compelled to remember them somehow, these people whose actions in the 1970s have shaped almost every aspect of my current life. Thanks, Dave.