Randall Hyde's Art Of Assembly Language is a highly regarded book which serves as both an introduction to assembly language programming concepts in general, and a tutorial for Intel x86 assembly language in particular. Randall has made it available for download for personal use at the link above.
John Fine's Home Page. John is the moderator for the newsgroup comp.lang.asm.x86 and has quite a bit of useful information on his site.
Robert Collins' Intel Secrets Web Site. Daily news about the Intel x86 computer industry. In addition Robert provides various useful information on programming Intel processors such as a protected mode tutorial, identifying CPU type, undocumented instructions, and others.
Christian Ludloff's Sandpile.Org site proclaims itself to be "The world's leading source for pure technical IA-32 processor information", and it probably is, although it doesn't really cover all of the 32 bit x86 compatible processors. All sorts of useful information on registers, instruction formats, and other details on processors from Intel, AMD, and other x86 processor manufacturers. Christian's Links Page has an incredible number of links to chip manufacturers, tools, books, hardware and software web sites, and tons of others.
Iczelion's Win32 Assembly HomePage is a must for anyone trying to do x86 assembly language under Windows.
Intel Developer Home is the main entry to Intel's web site for development and programming information. Intel Architecture Processor Home takes you directly to the main page for the x86 processors.
The ultimate guide to all of the interrupts used on PCs by DOS, BIOS, hardware, and hundreds of software packages is available in Ralf Brown's Interrupt List, a must have for any serious PC assembly language programmer.
You can view the Frequently Asked Questions FAQ for the newsgroup comp.lang.asm.x86 here.