If you want to access your locally-hosted site (e.g. running MAMP Pro or something similar) on any device on your network–including mobile devices–add an alias to your site with the following format:
Replace sitename with any URL-friendly phrase you want (I choose my site's name), and 192.168.1.1 with your local IP. Voilà! Instant multi-device testing, no extra software required.
Xip.io is a DNS forwarding service provided for free by 37signals, for use in exactly this situation.
(From this article)