AsmGen, 6502 assembly code generator for sprites, fonts and images (Apple II and Atari). Written in Python with documenation. There are alot of other interesting repositories on his site also
asmMerlin65816.vim, MOS 6502,65c02,65816 Assembly syntax + Merlin Add-Ons
Atalan, A fun language compiler for 8-bit systems (6502, 6510, Z80, Atari, Commodore, NES, ZX Spectrum, Atmos) with great optimization
cc65, A freeware C compiler for 6502 based systems
easy6502, Easy6502 by Nick Morgan is one-stop accessible tutorial on 6502 assembly language programming, including a series of worked example programs which you can edit and run in the embedded emulator
gcc-6502, A port of GCC to the 6502 processor family
llvm-65816, This is an attempt to add support for the WDC 65816 to LLVM. Don't even try to use it yet
Merlin32, Merlin 32 is a multi-pass Cross Assembler running under Windows, Linux and Mac OS X targeting 8 bit processors in the 6502 series (such as 6502 and 65c02) and the 16 bit 65c816 processor
westerndesigncenter, tools to support ROMable embedded code and in–circuit debug of control and application software
wudsn-ide, WUDSN IDE is a free integrated Apple II, Atari 2600, Atari 7800, Atari 8-bit, C64 and NES development plugin for Eclipse
x86-to-6502, Attempts to translate x86 assembly into mos6502 assembly
XA, xa is a high-speed, two-pass portable cross-assembler. It understands mnemonics and generates code for NMOS 6502s (such as 6502A, 6504, 6507, 6510, 7501, 8500, 8501, 8502 …), CMOS 6502s (65C02 and Rockwell R65C02) and the 65816
xa65, xa65 is a 6502 cross assembler, running on Unix (and Unix-alikes), and producing 6502 and 65816 code. It comes complete with a set of tools
xasm, 6502 cross-assembler with original syntax extensions
yagsdisasm, Apple IIGS / 65C816 disassembler written in PHP
x6502, x6502 is an emulator for the 6502 class of processors. It currently supports the full instruction set of the 6502 (plus a few extensions) and has a rudimentary simulated I/O bus
Hardware
C256, The C256 Foenix is a new retro computer based on the WDC 65C816 running @ 14Mhz
commanderx16, Commander X16 8-bit computer with emulator
bravo, OMEN Bravo / 65C02-based single board computer
C256-kernel, Functional specification for the C256 system ROM
FoenixIDE, Development and Debugging Suite for the C256 Foenix Family of Computers
neon816, This project is about the design of an 8-bit/16-bit nostalgia motherboard using currently in-production parts, and designed to interface primarily with modern peripherals