http://www.net-lettings.co.uk/  

A great comprehensive site - showing areas and general rents to expect in those areas. Covers a number of letting agents throughout the London area.

http://www.arla.co.uk/

If you're looking for a flat outside of London, this site shows a comprehensive listing of the "Association of Residential Letting Agents" registered operators.

http://www.hol365.com/

A good listing of letting agents throughout the UK. An area of http://www.homes-on-line.com that covers home buying as well.

http://www.leaders.co.uk/

Covers the Southern UK and West Midland areas. (Not London)