Watches your source tree for changes and runs a command when it does. Looks at filename, file size, and file mtime to determine what constitutes a "change". Saving a file with no other changes will cause it to re-run the command (I consider this a feature).

runwhen.py

#!/usr/bin/env python

import os
import sys
from time import sleep

usable_files = (
    '.py',
    '.php',
    '.yaml',
    '.json',
    '.rst',
    # Add file extensions you care about here
)

if len(sys.argv) < 3:
    print "Usage: {0} PATH SOME COMMAND TO RUN".format(sys.argv[0])
    print " e.g.: {0} . nosetests -v".format(sys.argv[0])
    sys.exit(1)

path = sys.argv[1]
execute = sys.argv[2:]


def get_usable_filenames(path):
    for root, dirs, files in os.walk(path):
        for filename in files:
            if filename.endswith(usable_files):
                yield os.path.join(root, filename)


def get_file_hash(filename):
    info = os.stat(filename)
    return filename, info.st_size, info.st_mtime


def get_state(path):
    return set(map(get_file_hash, get_usable_filenames(path)))

new_state = get_state(path)
old_state = None

while True:
    if old_state != new_state:
        print
        old_state = new_state
        os.system(' '.join(execute))
    else:
        sys.stdout.write(",")
        sys.stdout.flush()
        sleep(1)
    new_state = get_state(path)