For UK coins collectors. The values are in UK Pounds but it's main use I have found is it's complete listing of all UK coins by type. It's pretty comprehensive and nice to have in a single location.
For the benefit of non-UK resident collectors, I'll add that the prices quoted present a fairly good benchmark when buying a coin, prices will vary, but they give you a sensible idea of the sort of money you should expect to pay retail, and as such the approximate value of an old English coin involved in a swap.