Lightspeed's Reference Page is oriented toward Windows 3x users, but it also links the best DOS resources on the web. This is the page I come back to again and again when I want to branch out on a quest for software or information.
Short Stop's Free Software for DOS is an excellent site for information about software as well as downloads.
The File Library at Channel 1 is where I go when I want to pore through a DOS collection that is both vast and well organized. You will find lots of unusual programs, such as Graffiti Artist, a paint program used to create the background image on these pages.
Dr. DOS Betamax has an excellent site with lots of information about how to use DOS as well as a comprehensive list of links. His batch file tutorial is a good starting point for anyone who is just becoming interested in DOS and wants to find out what the techies mean when they describe DOS as "powerful." The site's author, Richard Bonner, is active in DOS groups on Usenet.