Techradar posted a relatively accessible and comprehensive history of the PC motherboard this weekend. The articles starts with the motherboard's IBM origins in 1981, and from there hits upon the major landmarks, including the introduction of the classic PCI expansion slot, all the way up to multigraphics card support and the built-in operating systems you find today.
For the most part we find the article provides an informative background, but the biggest news of the past few months isn't mentioned. At Nvidia's Nvision 08 conference in August, Nvidia announced that it will finally let Intel chipsets support Nvidia's SLI multigraphics card specification without also licensing a special chip. Starting with Intel's forthcoming Nehalem-supporting X58 chipset, we expect that motherboards supporting both SLI and ATI's already open Crossfire technology will be widely available. The result is that PC gamers will have a much wider range of upgrade options, and hopefully lower prices on SLI-supporting motherboards (although not with the X58, which we hear will be a very expensive high-end part).