Official sites of owncloud: http://owncloud.org  

OwnCloud is an open source software. The Server software and PC clients are free. Android Client charges HKD6.68 in Google Play (Sep, 21, 2012)

It is a DropBox in your own server. Installation is simple. It can syn your files in your hard drive. Many other apps can be added.

Installation Guide

Platform: Ubuntu Server 12.04.1 LTS with LAMP installed.
owncloud version: 4.5.0

  1. Install owncloud with aptitude with the following command.(sudo apt-get install owncloud)  It will install owncloud 3.0 
  2. change to the root directory of web server (cd /var/www/)
  3. Download owncloud 4.5.0 (sudo wget http://mirrors.owncloud.org/releases/owncloud-4.5.0.tar.bz2)

To be continued.