You can get an external case with power supply for very little - I got one in a sale a couple of weeks ago from a local PC store for £15.
You mount the internal drive into the casing then connect it up. Build the casing together and plug it into your USB or Firewire port and to a power outlet and off you go.
You'll get a warning if using USB 1.1 about having a USB 2 device plugged into a slower port, but it works fine at all speeds.
Search on Dabs .com for drive case and you'll find some for 2.5, 3.5 and 5.25 hard drives at £14, £26 and £34 respectively.