Foxtons
good for London including lofts. 360 degree views of some properties.
Hamptons
good for London, but online details don't give sizes
HomePages
Property search, mortgage advice etc, integrated site
MousePrice
Free land registry prices, estimated valuations
Property Vision
"The Buyer's Adviser": work for buyers, don't sell themselves
Right Move
Partnership with Halifax? Good glossary of terms.
The Modern House
Estate agent specialising in architecturally interesting properties
Winkworth
Excellent details including floor plans
WowHaus
Interesting period houses for sale
Zoopla
Free land registry prices, estimated valuations