65xx

a2DMAMagic
Make a secret, hidden, high-throughput, transparent 6502 DMA channel, used in Apple II VGA video card; circuit flags dead cycles, makes them available as DMA channel; made for Apple II, but usable on any 6502 system, can be added with no architectural modification.
http://homepage.mac.com/jorgechamorro/a2things/a2DMAMagic/

6502asm.com
6502 compatible assembler and emulator in JavaScript; program and run 6502 assembly in web browsers.
http://www.6502asm.com/

Dan B's 6502 Tech Page
Information, links, and programs on 6502s.
http://atarihq.com/danb/6502.shtml

6502 Development
Insights on early embedded processor development; with a 6502 wall chart, in PDF.
http://ericclever.com/6500/

6502 Emulators
List with table, descriptions, links. Zophar's Domain.
http://www.zophar.net/6502.html

MOS Technology 6502 Microprocessor Family
Short technical description, photographs, links to family members.
http://www.cpu-world.com/CPUs/650x/

Wikipedia: MOS Technology
Encyclopedia article with links to many related topics.
http://en.wikipedia.org/wiki/MOS_Technology

Wikipedia: MOS Technology 6502
Encyclopedia article with links to many related topics.
http://en.wikipedia.org/wiki/MOS_Technology_6502

650x
Short technical description, photographs, links to family members. CPU-collection.de.
http://www.cpu-collection.de/?tn=1&l0=cl&l1=650x

The 6502 Gets Microprogrammable Instructions
Shows hardware method to add 64 user-defined instructions to a 6502; in text, tables, block diagrams. BYTE. <small>(October, 1980)</small>
http://www.wiz-worx.com/resume/byte8010.htm