Python, do something like:

import gnumeric

# Simple function to add two numbers.
def foobar(a,b):
  return a + b

gnumeric.register_function("adder", "ff", foobar)

Use Makefile.temp to compile the plugin for now until detection of Python
path is put into configure.in. Python code will be
loaded from ~/.gnumeric/main.py so stick the above Python code snippet
in that file to see it run.