National Biodiversity Network Gateway
This is a site that I use often, mainly to check the recorded distribution of species I find. It’s very easy to use – just put the species name in the search box and a map of the UK comes up showing distribution. For some data sets it is possible to see records in 100m squares – very useful.
There are some limitations. The most obvious being that the maps are only as good the data provided. For example, I live in Lincolnshire and this appears either to be extremely under-recorded, or records have not been added to the NBN database. Many insects I find quite commonly in my local patch are absent from the NBN maps for this area.
This is a site that is likely to become more and more useful as time goes on and records are added.