A free app for Android, by webrox.

LAN drive is a free app that lets you turn your phone into a network drive. It works on all devices with LAN (local area network) interface. It supports all major operating systems, and is extremely easy to use. With it, you can share files between multiple devices on the same LAN network, or between different LAN networks. It also provides an easy way to backup your important data.

How to use it:

The first thing you need to do is install the app on your phone or tablet. After that, you will be able to turn it into a drive.