A very, very small (and cheap!) USB Bluetooth dongle, which works out of the box in Linux.
0a12:0001 Cambridge Silicon Radio, Ltd Bluetooth Dongle (HCI mode)
This is my entry for this week's Photo Friday challenge: "Professional".
No it does not work OOTB.
Not even ITB...
Sitecom CN521 does however...
Which USB ID does your device have? Which kernel did you use and which tools to test whether it works? It sure worked fine for me (I tested with hciconfig / hcitool, and by attaching a BT-GPS device successfully).