An exhaustive, well illustrated compilation of the British leafminers: British Leaf Mining Fauna.
The leaf and stem mines of British flies and other insects UK Flymines
The site covering all Microlepidoptera of the Netherlands (also the non-mining species): microlepidoptera.nl.
Sources of pictures of moths and larvae: UK moths and lepiforum.
Some more general, insect-related pages are the sites of the Netherlands’ Entomological Society and the Flemish Entomological Society.
For nomenclatural matters the Fauna Europaea and Markku Savela’s website are invaluable, and for the status of all insects in the Netherlands we have the Dutch Species Catalogue