James Derrick Slater wasn't a professional chess player, but at one brief but very significant moment he was one of the most important figures in the chess world. At a point when Bobby Fischer was waffling about playing his 1972 world championship match with Boris Spassky, British businessman Slater put up $125,000 of his own money, doubling the prize fund and in effect telling Fischer to put up or shut up. Fischer decided to play (at least except for game 2), and the rest is history. Slater subsequently offered cash incentives to the first four British players to achieve the grandmaster title (Tony Miles was the first, and received the largest bonus) - just the sort of help British chess could use nowadays.
For more about Slater, here is the basic Wikipedia entry, here's a piece on his love of chess and contributions to the game; finally, here is an obit covering his life as an investor and businessman. (HT to David McCarthy for the news and the latter two sources.)