386SWAT - A Protected Mode Debugger which provides debugging services to real mode, protected mode, virtual 8086 mode programs, DPMI and VCPI clients, and Win 3.1x and Win95/98 programs under any memory manager while occupying only a few kilobytes of low DOS memory - http://www.sudleyplace.com/swat/
Programming MS-DOS with Power - Information on most every aspect of DOS programming. - http://www.frontiernet.net/~fys/index.htm
comp.os.msdos.programmer FAQ - Freqently asked questions about programming for MS-DOS with tested answers. - http://www.jeffc.org/msdos/dos-faq.html
UPX - The Ultimate Packer for eXecutables is a portable, extendable, high-performance executable packer for several different executable formats. Freeware. - http://upx.sourceforge.net/
X2FTP Archive - General DOS programming material. - ftp://ftp.cdrom.com/pub/x2ftp/
Ralf Brown's Interrupt List (USA Midwest mirror) - A list of interrupts used on the Intel x86 architecture and their meaning with over 9000 linked pages and 350 indexes greatly ease the process of searching. - http://ctyme.com/rbrown.htm