The other day I wanted to watch a movie, but I realized I'd left it on a hard drive, and it would take too long to go upstairs and copy it onto my computer, and I won't be able to stream it to my devices. So I decided to make myself a home server, I could access all the files on a hard disk wirelessly anytime, anywhere. after a 2 mornings, an afternoon, and 4 hours of procrastinating on my homework (long live the maker) and I was able to realize my want.  
I know it doesn't look too much more than a PCB on a hard disk. but it's mostly in the software. That's where the magic happens. This setup is like a personal cloud, with little work, I can pull in and pull out data from it through my home router. This setup consumes far less power than a conventional server. Swap the hard disk drive with a solid state drive and you're pushing beneath 3.5 watts!
just add a 10000mAh battery and you're looking at a portable router that can go 14 hours unplugged!

All you need is a computer running an SSH software such as PuTTY (link in the end)
so you can remotely switch the server on and off, an SD card with an OS such as Raspbian (link below) and of course the Pi and a USB storage device.

 
Here's the PuTTY configuration screen.

Here's a screenshot of the Pi terminal. Simply install a few services, just like this:
sudo apt-get install apache2

sudo apt-get install php5

Then, just restart the services and install mysql, and owncloud (links below) from their websites, and run those.

After a little work, you should be able to access your files on this link
[your ip address without brackets]/owncloud

Configure your login to secure your data and you're done.

Links:
https://owncloud.com/download/
www.mysql.com/downloads/
https://www.raspberrypi.org/downloads/
www.putty.org/