It's very easy to write one in Python... Anyway, it's already written, you just launch it.
import sysIt serves the contents of the current directory. You can give it an argument like
import BaseHTTPServer
import SimpleHTTPServer
listen = ":8080"
if len(sys.argv) > 1:
listen = sys.argv[1] + listen
listen = listen.split(":")[:2]
listen[1] = int(listen[1])
listen = tuple(listen)
httpd = BaseHTTPServer.HTTPServer(listen, SimpleHTTPServer.SimpleHTTPRequestHandler)
httpd.serve_forever()
localhost:8080, 0.0.0.0 or :8000 to listen on different IP/port. But be warned that this simple stuff is very insecure!