eForth is a Forth system written by Bill Muench and designed to be easily ported. The download available here contains ports for the 6502 family.
For more information about eForth, please visit Bill Muench's eForth site linked above. Some documentation can be found in the file eF/docs/readme.eForth.txt in the eF22a-dev.zip (a port for the GreenArrays, Inc. GA144 processor) archive file there. Note that the 6502 family ports here were based off of an older version of eForth (specifically EFORTH.X86) so it differs from the GA144 port in places.