Www.plexapp.com
This is a beta but for all that its well developed and well supported by volunteers and developers alike. You can download the media server software as well as desktop media client software for free (windows, Mac and Linux) plus there's a free cloudlike service that makes your server accessible to paid mobile clients (like the $4.99 Plex for Android for kindle fire).
Basically if you leave your server running, you can access your media from anywhere on virtually any device without having to upload your media and pay for cloud storage. The content stays on whatever drive you store it on.