Crucial external Caddy

Depends if you want USB3 or Thunderbolt, and what you want to do with it.

USB3: http://eshop.macsales.com/item/Other World Computing/MOTGS3U3/ - I have 4 of these used with various HDDs.

Thunderbolt: http://eshop.macsales.com/item/OWC/MOTGTBH5T1.0/ - I have two of these used with SSDs - I took out the HDDs supplied and put it into USB3 enclosures as above

 

The most important thing for a USB 3.0 is that the enclosure support UASP (USB Attached SCSI Protocol). UASP delivers much higher transfer rates for SSDs because it's a lot more efficient than the standard USB mass storage protocol. (I believe there's still some benefit with hard drives, but it's not as dramatic.) Note that you may need to connect the enclosure directly to a USB port on your iMac to get the benefits of UASP, since UASP isn't always available when you're connected through a hub.

I don't see any mention of UASP in the specs for the USB3 enclosure that matreya links. I've got an Inateck aluminum enclosure with a Samsung EVO 850 1 TB in it, and it's working very nicely on my Retina iMac. I get approximately 427 MB/sec read and 360 MB/sec write speed with it (according to Blackmagic Disk Speed Test).