Libya is a cash society. Credit cards are not widely used, although some are accepted in major hotels and banks. You can walk into some of the major banks and ask if it is possible to withdraw cash against your credit card. Recent reports from travellers suggest that this is possible in Benghazi and Tripoli, although it may depend on individual cases and the information is liable to change.
There are working ATMs in Tripoli, Benghazi and some other towns. In the capital, head to Martyrs' Square, where you'll find branches of the major Libyan banks. Visa and Mastercard are accepted; American Express may also be in some cases.
Some ATMs in Libya have built-in MP3 players that blare tracks while you wait for your money. There are also mobile ATMs, attached to the back of small vans, that move around Tripoli, parking in various places; again, try the area around Martyrs' Square if you're looking for one of these.
Traveller's cheques are generally not accepted anywhere in Libya. Western Union operates in Tripoli, Benghazi and some other towns. Check the company's website for listings of providers.
Sat-Thurs 0700-1400 and 1600-1800. Most banks are closed on Fridays, Sundays and on Saturday afternoons.