Relauch64 is a cross-platform IDE for Commodore 64 available for Mac, Windows and Linux, and the developers behind the project just released a new version 3.3.6.
This editor works together with common cross assemblers. The supported assemblers are 64tass, ACME, ca65, DASM, DreamAss, Kick Assembler and TMPx. Other assemblers might work as well, but syntax highlighting may not be 100% correct.
Link: Relaunch64