Feng Office is Open Source software. You can download the free and Open Source Feng Office Community Edition, to install and run on any compatible server, to use it at no charge, forever.
In order to install Feng Office on your own Server you need to correctly setup a Web Server (We recommend Apache) with up to date PHP, and an up to date MySQL database with InnoDB support.
If you just want to try out Feng Office you can sign up for a free account here.