Identification, attribution, and comparison
CNG's archives -- usually my first stop when trying to identify a coin
Tesorillo-- fantastic site for identifying late Roman bronzes
Collectors' websites of note
Colosseo Collection-- View at your own risk. Your coins may seem unsatisfactory after ogling this astonishing collection!
Doug Smith's Ancient Coins-- If you've spent any time on CoinTalk or FAC's boards, you know Doug. He's a great source of coin knowledge, presented with wonderfully dry wit.
Warren Esty's site-- difficult to navigate but it contains quite a bit of good information
Man-Faced Bulls-- Nicolas Molinari and Nicola Sisci are utterly obsessed! Their website is comprehensive and continually updated. The link is to the main page (bronze coins); the landing page has links to their separate sites for gold and silver MFB coins.
Forum Ancient Coins discussion boards-- one of the oldest ancient coin boards, full of in-depth information, frequented by several well-known numismatists. Downside: slow-loading; chock full of rules which can be intimidating for new users