Community Trade Marks On-line (http://oami.europa.eu/CTMOnline/RequestManager/de_SearchBasic?transition=start&source=Log-in.html&language=en&application=CTMOnline)
Database of European Community Trade Marks.

UK Intellectual Property Office Trade Marks Databases (http://www.ipo.gov.uk/tm/t-find/t-find-text.htm)
Covers UK trademarks only.

International Trade Mark Classification (http://www.wipo.int/classifications/en/)
The Nice Agreement classification scheme covering the kinds of products and services being trade marked.

Other trade mark databases and information websites (http://www.bl.uk/collections/patents/tmlinks.html)
Many other sources of information on trade marks available from Information page provided by the British Library.