Did you know iFixit was good for something other than posting galleries of new gadgets they've torn apart? Well they are, and their latest good deed involves devising a kit so that you can install a second hard drive in your mid-2011 iMac.
When iFixit tore down the iMac after the last refresh, they found mounting points which were completely unused. So they built a kit that lets you place an additional hard drive in that spot. For $70, each kit comes with the necessary SATA cables, mounting tape, suction cups (for removing the screen), a driver kit, and a spudger.
iFixit recommends adding a smaller, cheaper SSD and using that as your boot drive while using your HDD for media files and things of the like. Hard to go against that advice. [iFixit]