I don't use these too often, because its a tough place to start research. To search a database, you need to already know the proper terms to look for. But they do come in handy from time to time:
http://numismatics.org/collection/accnum/list - the collection of the American Numismatic Society
http://www.celticcoins.ca// - a database of Celtic coins, complete with Van Arsdell, Alan, and Mack reference numbers
http://www.sylloge-nummorum-graecorum.org/ - Database of coins in a number of private and public British collections; includes coins from the Fitzwilliam and Ashmolean.
http://www.ukdfd.co.uk/ukdfddata/index.php?cat=1 - A useful database for coins found by metal detectorists in the UK. Everything from Celtic staters to Victoria sovereigns. (Probably a great place for tokens, too, Toad!)
http://www.fitzmuseum.cam.ac.uk/coin...emc_search.php - a catalog of coin finds of coins of the early Middle Ages. Put together by the Fitzwilliam Museum, Cambridge.
http://rpc.ashmus.ox.ac.uk/ - The Roman Provincial Coinage database. Sponsored by Oxford, its the online face of project to publish all provincial coins in a single series, as a companion to the ten Roman Imperial Coinage volumes. So far, Volumes I (Julio-Claudians) and II (Flavians) have been published. As well as a section from Vol VIII on Gordian III in Asia (in French. Now that's just silly). This is the section that they are currently working on, and they're uploading all of the entires to this database. Currently, it only covers the Antonine era, from 138-192 AD.
http://www.cngcoins.com/Coins_sold.aspx - The database for coins sold by the Classical Numismatics Group. Unfortunately, they usually don't record the legends. But they do give very specific references and almost always include pictures.
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