BUG
Selected budget and backpacker accommodation options in London rated and reviewed by guests.
http://www.bugeurope.com/reviews/England/London/West_End/

Fodor's
Contains detailed descriptions of well over 100 hotels in the city.
http://www.fodors.com/london/

Frommer's
Read page-long write-ups of 100+ hotels in the city.
http://www.frommers.com/destinations/london/55_indacc.html

Hotels England.com
London hotels reviewed by professionals and rated by travellers.
http://www.hotelsengland.com/allhotels/London

Johansen's
Johansen's offers detailed reviews of dozens of hotels in London and the surrounding area.
http://www.johansens.com/

Time Out
The site offers detailed descriptions of London accommodation in various price brackets.
http://www.timeout.com/london/hotels/

Trip Advisor
Offers customer reviews of dozens of hotels in the city.
http://www.tripadvisor.com/
http://www.tripadvisor.co.uk/

Wcities
Read customer reviews of hotels in the capital ranging from deluxe to inexpensive, or post a review of your own.
http://www.wcities.com/en/cat/23/1/category.html