English Language:

www.worldcoingallery.com:   Don's World Coin Gallery, You can find almost every Modern Coin here. 
www.ukcoinpics.co.uk:   A lot of pictures of British coins. Also a lot of relevant information about british coins. 
www.spink.com:   Dealers and Auctioneers in Coins, Medals, Stamps, Banknotes, Books, Bonds & Shares since 1666 
www.belgjos.tk:   A good overview of Belgian Error Coins and Essais 
www.coinsgb.com :  With over 250 colour pictures and over 200 pages of information.

This website is designed to be a reference and help British coin collectors
identify various milled coins minted from Oliver Cromwell to George VI. 

Dutch Language:

www.muntstukken.be:   Good information about Coins from The South Netherlands (Zuid Nederlanden) (1419-1794) + Good Forum about all coins. 
www.belgjos.tk:   A good overview of Belgian Error Coins and Essais. 
www.numismaticabelgica.be:   New belgian catalog. Catalog language: Dutch/French/English