names.co.uk is an English registrar and web hosting company.

Their frames-based hosting option has an XSS vulnerability allowing injection of arbitrary Javascript.  For example:

http://www.weddingbiz.co.uk/%22%3E%3Cframe%20src%3D%22javascript%3Aalert%281%29%22%20

According to webhosting.info, names.co.uk hosts 75k domains.  If even a few percent are using the frames-based hosting option, then this vulnerability would affect thousands of sites.

This vulnerability was reported twice to names.co.uk in early September 2005 (ticket SU197304).  Their response was that it was not a security risk.