Embedded Systems Programming
Embedded systems sight, includes archive of code from Embedded Systems magazine.
Large site with many documents about the PC. Not directly about assembly, but much low level information that is useful can be obtained here.
Ralf Brown's Homepage
Ralf Brown's Home page. I put it here since he's more popular in the Assembly language crowd.
Ralf Brown's Interrupt List
The INT list in HTML format with search engine.