Here are some websites that I think are useful for anyone planning a visit to London:
Home | Transport for London - Useful for planning journeys by public transport as has up to date information about delays any problems. Also has tube maps and information on ticket prices.
Days Out Guide - London attractions with train tickets | London trains | trains to London - Lists many attractions with discounts like two for one if you travel by National Rail.
Streetmap - Homepage - Online map of the UK. Useful if you want to plan how to get from a station to the attraction.
Visit London - the official website for London - explore now - Once again lists attractions, hotels etc. in London.
Hope these help and feel free to add any more you find useful. 