Currency
The Euro: €
Ministère de l'Economie, des Finances et de l'Industrie

ATM
Visa, MasterCard, Eurocard (any bank with the CB sign will do)
AmEx (a few in the very large cities and everywhere else via Le
Credit Lyonnais Bank)
Cirrus (via Le Credit Mutuel and Le Credit Agricole)
It will be very easy to get cash with Visa, MasterCard and Eurocard possible with AmEx, difficult with Cirrus and impossible with Plus.
Credit cards
Accepted everywhere. Look for the sign CB in green-blue Visa, MasterCard, Eurocard (Carte Bleue network) AmEx, JCB (sometimes, and nearly always where there are tourists)
Travelers checks
Easy to cash at any bank. There are often charges...
AmEx, Visa and Thomas Cook work
Post-Checks are cashable at every Post Office.
Foreign exchange
Paris: Best rates are usually given at banks or at the American Express office (near the Opera). The best rate you can ever find is La Banque de France, but the line is so long that you need to change a lot of money to make it worth. Another fair bet is Comptoir Change Opera. In Paris, there are change offices open at any time everywhere tourists go (bad rates, but no commission ). Nearly every currency can be changed.
Rest of France: definitely banks. It will always be easier to change something into euros than euros into something since banks have a limited stock of foreign currency.