American-British English
Two countries divided by a common language, it is commonly said. Well, here's the proof.
http://www.holiday-inn.co.uk/ISayTomato.shtml

British Airways.com
Learn to speak like a Londoner using BA's Brit-Speak Dictionary.
http://london.ba.com/
http://www.ba.com/know/

Cambridge Dictionaries Online
An online English dictionary.
http://dictionary.cambridge.org/

CILT.org.uk
The website contains details of what foreign languages are spoken in London and by how many.
http://www.cilt.org.uk/faqs/langspoken.htm

Fodor's
Essential phrases covering topics such as dining, airports, asking directions, health, shopping, and more in English, French, German, Spanish and Italian.
http://www.fodors.com/language/

Learn English
A free site run by the British Council that will help you learn English.
http://www.learnenglish.org.uk/

London Cockney Rhyming Slang
Common slang and Cockney rhyming slang.
http://www.timeshare.freeserve.co.uk/tourism/lon/slanga.html

Microsoft
Online dictionaries made up of local British dialects.
http://www.microsoft.com/uk/dialect/default.mspx

Moto Europa
A European phrase book for motorists; just click on the appropriate link under the column marked "on the road", on the main page, to access the information.
http://www.ideamerge.com/motoeuropa/phrasebook/chapter/index.html

Smartphrase.com
Lists essential words and phrases for different scenarios ranging from ordering a meal to visiting a bank, in a handful of popular European languages, English included.
http://www.smartphrase.com/

The English to American Dictionary
An explanation of words used on both sides of the Atlantic, but with very different meanings.
http://www.english2american.com/