GeckOS, Unix on 6502:
http://www.6502.org/users/andre/osa/index.html

eater.net: 6502 kit
https://github.com/dpm-343/6502-monitor/blob/master/6502-monitor.ino

other 6052:
http://lateblt.tripod.com/6502prj1.htm
74LS138 decoder IC (3to8)

XA cross assembler
http://www.floodgap.com/retrotech/xa/xa-old.html

http://srecord.sourceforge.net/
VIM xxd (hex to bin and vice versa)

TL866II+ eeprom programmer
https://gitlab.com/DavidGriffith/minipro/

other projects
http://www.stuartconner.me.uk/tms9995_breadboard/tms9995_breadboard.htm

differences 6502 and 65c02:
http://www.6502.org/tutorials/65c02opcodes.html

https://jacob.jkrall.net/ps2-on-65c02
PS/2 keyboard

interrupts:
http://www.ganssle.com/articles/anmi.htm

UART:
https://www.jameco.com/z/6551-Major-Brands-IC-6551-Asynchronous-Communication-Interface-Adapter-DIP-28-pin_43318.html

assemblers for 6502:
http://sun.hasenbraten.de/vasm/

emulators for ben6502:
https://github.com/alimansfield2016/ATM65C02_EMU, nice idea, c++ code quite bad
https://github.com/alimansfield2016/ATM65C02_EMU.git, sadly in Javascript
https://github.com/chanmix51/soft65c02, sadly in rust

other/todo:
https://news.ycombinator.com/item?id=22077052
https://www.instructables.com/DIY-Paper-TapePunch
http://wilsonminesco.com/BenchCPU/ (VIAs for all kind of external stuff)
https://github.com/mist64/msbasic
https://www.pagetable.com/?p=46

documentation on old 8-bit logic:
http://www.zimmers.net/anonftp/pub/cbm/documents/chipdata/index.html

Geck/OS:
http://www.6502.org/users/andre/csa/
http://www.6502.org/users/andre/osa/index.html

OS:
https://github.com/janroesner/sixty5o2, uses Arduino and serial for loading programs,
  good example of a bootloader assembly code, interrupt handling
http://www.6502.org/users/andre/osa/oa1.html: OS/A65
http://danceswithferrets.org/geekblog/?p=808 (monitor, http://exifpro.com/utils.html, emulator)

7-segment:
https://www.robotshop.com/community/forum/t/explaining-the-7-segment-display-and-74hc595-shift-register/13139
https://www.eevblog.com/forum/projects/7-segment-hex-bus-display-with-eproms/
https://www.youtube.com/watch?v=BA12Z7gQ4P0

6502 links:
https://www.eiroca.net/wiki/archive/awesome/6502
http://wilsonminesco.com/

practical stuff:
https://www.instructables.com/Fix-a-broken-pin-on-an-IC-chip/
DRY ZIF with paper and tape (george foot)

other projects:
http://minus56.com/
https://www.chrisward.org.uk/6502/dev.shtml
https://www.heiko-pruessing.de/projects/hbc65/
https://c74project.com/ (a full 6502 in 74xx logic gates on PCB)
https://github.com/Klaus2m5/6502_65C02_functional_tests (format tests and verification)

delay/timers:
https://gist.github.com/superjamie/fd80fabadf39199c97de400213f614e9
http://6502org.wikidot.com/software-delay
http://wilsonminesco.com/6502primer/PgmTips.html
http://6502.org/source/general/clockfreq.htm
http://wilsonminesco.com/6502interrupts/ (use VIA and NMI to produce a 10ms timer)

software archives:
http://6502.org/source/

os:
http://6502.org/source/kernels/minikernel.txt (timer and time slicing)

apple emulator:
https://damian.pecke.tt/2015/04/06/turning-the-arduino-uno-into-an-apple-ii.html