Assembly language has become kind of a dying art these days. There really aren't very many good sites dedicated to Assembly Language programming any more. Here are a couple of places where you can start, however.
Eric Isaacson's Home Page has the latest version of the shareware A86 Assembler and the D86 Debugger. I use the A86 assembler to compile all of my programs. It is MUCH easier to use (especially if you're a beginner) and MUCH faster than Microsoft's MASM. To compile My Source Code without modification, you'll need A86.
Programmers Heaven contains lots of source code in several different programming languages (not just Assembly Language). It also has several links to other pages with source code.
Doctor DOS Betamax contains some interesting and helpful information about DOS, including tips on Batch file programming. It also has some links to lots of other DOS sites.
WordPerfect for DOS contains all kinds of up-to-date information about getting WordPerfect for DOS to work with modern hardware (modern Video Cards and Monitors, Printers, etc.). The site is specifically directed at WordPerfect, but some of the information from the site can be used to get other DOS programs working as well.
Free Software for DOS is being maintained once again by Rich Green. He tries to keep track of (and review) the most useful DOS freeware he can find. He only lists programs he finds useful and stable, so it's a good place to start looking if you don't want to wade through dozens or hundreds of programs to find a good one. The amount of time he has spent simply reviewing the programs and and giving them a personal "star rating" makes it worth the effort to at least look at the site.
Interesting DOS Programs (as the name implies) has downloads and links to several interesting and useful DOS programs.
This downloads several versions of the IBM Technical References. I have downloaded these from various other places on the Internet, but they are a little difficult to find, so I have put them here if you want to use them. This file is really big (almost 50 MB), so make sure you have enough room on your hard drive. When there is a question or dispute about what some low-level function in the computer is supposed to do, the IBM Technical References are usually considered the "definitive" source.
This downloads an article by Chris Dunford that explains the the IBM Interrupt Sharing Protocol. The Protocol has been around for a long time, and all programs (especially TSR's) should use it. Unfortunately, not very many people know about it. I discovered the Protocol myself not too long ago, and will start using it in all of my Programs as I update them.