64tass
A cross-assembler for the 6502 family that runs on a PC but speaks the syntax of the C64's own Turbo Assembler.
64tass was originally written by Marek Matula of the group Taboo and has since been maintained and extended by others in the scene as open source. It handles the 6502/6510, 65C02 and 65816, and layers macros, conditional assembly, sections and a proper expression engine on top of the old Turbo Assembler syntax.
Together with ACME and Kick Assembler it makes up the trio that dominates modern C64 development. The workflow is typical of today's retro scene: write in an ordinary editor, assemble from the command line, launch the result straight into an emulator. Being able to feed it thirty-year-old source almost unchanged isn't a side effect; it's the point.