TimeMachine Backups - Justin showed us how to use a network share for TimeMachine backups last year, and the same technique will work using your new Mac fileserver too.

Running BitTorrent - Particularly useful if you're going to be streaming media from the same machine, Transmission is a great native OS X BitTorrent client with a fantastic web UI that you can access from your smartphone, tablet or any other web browser (just paste in your magnet link and go).

Personally I use PS3 Media Server for streaming and transcoding content to local devices, but you might find that your old Mac is one of the best devices to use as a Plex media server. Buffalo TeraStation 1200D Buffalo makes one of the best looking NAS devices on our list, fitting well within. Streaming Media - Why not share your videos, music and pictures too? iTunes HomeSharing works for this, but so do other free network streamers. The TerraMaster F2-220, without drives, can be found on Amazon and Newegg for 199.99.

Aside from connecting all of your spare external drives and creating a mountain of reliable, easy to manage network storage using your old Mac, you might also be interested in some of the following:

Nas for mac system