Some of the most useful sites I have come across in my day to day research:

http://ononemap.com - Useful for checking properties that are for sale and for rent on an actual map to compare with a specific location. Other sites tend to give you 1/2 mile radius and the type of properties can vary significantly within just 1/2 mile so sometimes better to look at specific streets or blocks.

http://rightmove.co.uk - This site pretty much compiles all estate agents properties for sale and to let. It allows filtering down to first 3 letters of a postcode so can give alot of variations that can confuse comparisons, however, it is perhaps the most comprehensive site.

http://upmystreet.com - A very useful site for not only gathering existing home selling prices and rents, but also for checking demographic information on areas such as: crime statistics, population breakdown, schools in the area etc.

http://eigroup.co.uk - This site is an absolute MUST if you are looking at buying at auction. Rather than having to order, gather and sift through hundreds of auctions catalogues, this site compiles all information on properties for sale throughout the country and allows you to set-up elerts which notify you of specific types of properties you are looking for in specific areas. There is a subscription service, however, with the time this site saves you it is well worth it!

http://riky.co.uk - This is a very useful site if you ever need advice on anything relating to landlord issues. It has a wealth of information and forms such as Assured Shorthold Tenancy's, Section 21, 8, 13  notices etc. A must for any serious landlord!

http://hometrack.co.uk - This is useful if you want to find historic house selling prices in a certain street or area.

http://www.summitfinance.co.uk - A UK based finance company well grounded in offering Below Market Value related financing products with some unique methods of structuring purchases to limit cash input. Well worth a look at!