Tremendous searchable database of of plants, annuals perennials, bulbs, shrubs, tree, herbs, climbers etc.
Find you plant by name, garden aspect, soil type, colour, size, season interest, hardiness, foliage, fragrance etc.
It also gives you names, maps and links to suppliers for that special plant you can't find in the garden centre.
The RHS Encyclopedia of Plants retails at £28 in Waterstones. The RHS What Plant Where is £20. So if you intend to buy these books you could be saving at least £48 by using the online search.
The RHS database which is free to search is not the same thing as the books mentioned above. You do not 'save £48' with the so-called 'freebie'.