A Page in Progress

Links are always problematical, because they have a habit of either moving or going away entirely. If you find that a site has moved (i.e., you are redirected to a different URL) or has vanished without a trace (i.e., the notorious "Page Not Found" error, please let me know by email (click on the webmaster link at the bottom of the page).

Wildwinds site has hundreds (maybe thousands) of pictures of different Roman coins, attributions, a search engine for coin inscriptions (with wildcard ability) and links to many useful sites. It is not a commercial site.
FORVM is a commercial site of a reputable dealer with many coins for sale. There are also many good articles. There is a major discussion group associated with it. He also has some supplies in his catalog. One valuable feature is the coins for sale do not go away when sold; the pictures and descriptions of sold coins are archived. That feature, along with other articles and guides, is quite helpful in attributing coins. There's also a fun coin cleaning contest.
Noble Roman Coins
Nobel Roman Coins is a commercial site of a reputable dealer with many coins for sale. He also carries supplies for collectors, especially cleaning tools not available elsewhere. There is a lot of information about coin cleaning techniques, including a very good tutorial download.
Moneta Romana
Moneta Romana, located in Europe, has Roman Imperial coins for sale. Be prepared to deal with some German, though. Payment is in Euros through PayPal only.