MB21 The Transmission Gallery
Great for looking at the commercial transmission sites for FM, DAB & TV.
(when searching for a specific station just google name of station & frequency followed by MB21)
Ofcom Site Finder
Great for looking up cellular communication sites on masts or tower blocks, also gives the height of the highest GSM panels. (Not always accurate)
Great for checking where UK commericial FM stations are broadcasting from and how much power they are using. You can download the list in PDF.
UK Free TV
Like frequency finder but also gives a basic coverage map when you click on the TX on map and various other useful information.
Radio Coverage Prediction
Great for giving you a rough idea of where your signal is going to cover and how strong from your chosen location, TX power, antenna height, gain, direction etc.
Don't say I never give you nothing!Emporis
Great for checking the height of tower blocks. (most London tower blocks are on the database)