Open DMX USB
Made by: Enttec
The OpenDMX USB is based on a FT232BM chip, a USB to Serial converter produced by FTDI. The widget is capable of DMX transmission and reception, but is totally reliant on the CPU to produce (or capture) the DMX signal, so this type of device is called 'unbuffered'. It works quite well, but if for example your PC froze for a second, so would the DMX. The design is GPL (you can build your own!) and is quite basic, and does not offer any protection for your equipment.
Note that receiving DMX does not currently work under Linux.
Linux Kernel 2.4: via DMX 4 Linux