The obvious ones first:
http://www.ntcompatible.com/
http://www.ece.mcgill.ca/~vromas/vdmsound/
http://dosbox.zophar.net/
http://vdmsound.sourceforge.net/compat/

DOS/Windows History
http://www.powerload.fsnet.co.uk/timeline.htm

For the inevitable:
http://www.windowsreinstall.com/

All kinds of info here, but it concentrates mostly on fine-tuning modern PC's to run DOS really well.
(Look at his powerhouse AUTOEXEC.BAT and CONFIG.SYS)
This is also where I found out how to use UMBPCI with Win98 to get 625K of base memory with DOS CD-ROM and mouse drivers loaded).
Also a lot of NT/2000/XP info, but he concentrates on Win9x.
http://members.aol.com/axcel216/

All of Sierra's adventure titles
http://www.sierra-quest.com/

Well-done sites for certain specific games:
Alternate Reality
http://w1.214.telia.com/~u21405572/alternate- … lity/index.html
Bard's Tale
http://www.geocities.com/thebardstale/
DarkLands
http://www.darklands.net/
Legacy of the Ancients
http://www.legacyoftheancients.com/

Adventures
http://www.adventurecollective.com/index.shtml
http://www.if-legends.org/%7Eadventure/
http://bioinfo.mshri.on.ca/people/feldman/vgm … seum/index.html
(^^^^^^^Awesome scans of maps, boxes, catalogs, trinkets, etc...)

Console stuff (really old stuff, that is)
http://www.tripoint.org/games/games.html
(I especially like the catalog collection)
http://www.tripoint.org/games/literature/cata … s/catalogs.html