yagsdisasm, Apple IIGS / 65C816 disassembler written in PHP
x86-to-6502, Attempts to translate x86 assembly into mos6502 assembly
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
Merlin32Language, Visual Studio language service for 6502 Merlin32 cross-assembler
Visual6502, Visual Transistor-level Simulation of the 6502 CPU and other chips!
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