Useful Links:

http://the-all.org/tx81z/index.html Self proclaimed to be to be the "Yamaha TX81Z Homepage" Good articles on FM theory and lots of other useful goodies for 4 OP users.
http://www.home.zonnet.nl/m.tarenskeen/yseditor.htm Legendary editor for Atari ST computers, lots of great resources.
http://launch.groups.yahoo.com/group/YamahaDX/ Yahoo group dedicated to DX. (Basically all Yamaha FM synths) Great group plenty of resources.
http://www.dxsynths.com/dxsynths/index.php A site is designed to help you quickly and easily find the latest Yamaha DX reviews, Yamaha DX parts, and Yamaha DX patches and Yamaha DX for sale.
http://120years.net/nav.html Site that documents the historical journey of electronic musical instruments. Grouped by decade, very interesting, all the usual synth saga's can be found here, ergo the story of Moog, Korg, Yamaha, Arp etc...
http://web.quick.cz/a.tom.x/dxage.html A whole host of individually profiled Yamaha FM synths are listed here, from DX7 to YS200. Not the most comprehensive site out there but there's a lot of "synth profile" data.
http://www.yamaha.co.jp/manual/english/index.php Yamaha's manual library, if we missed one here you'll find it in their catalog
http://sounddoctorin.com/synthtec/yamaha/dx27.htm The Synth Doctor. Plenty of technical info here as well as service info etc.
http://www.musictechnologiesgroup.com/ Backlit LCD's on older equipment.