For the UK Multiboxers who want something similar to X-keys

http://www.intolect.com/progkeyboards.htm

Seems to be more configurable (more layers) than X-keys, quite expensive though and I'm not sure if they are PS/2 or USB.

Also
http://www.ceratech.co.uk For a wide range of Mini-keyboards

and http://www.accesskeyboards.co.uk/ For a multitude of programmable keyboards