SunPlus released sources for the C compiler. It doesn't include the assembler and is based on gcc2 so quite hard to compile: https://github.com/vsmile-dev/unsp-gcc
XASM is the original assembler provided by the µ'nSP IDE
naken_asm has support for µ'nSP and provides a standard include file for SPG200 registers.
This early homebrew experiment took an interesting approach of not using an assembler at all.
MAME has good support for the V.Smile.
Unununium is a standalone emulator, but is known to have some bugs.