As the new iMac does not include a standard 56k model, Apple has released a external USB modem as an optional accessory. It's simple to use and looks quite nice, it's priced at $49.
Some more technical details -
- Speed: Data up to 56kbps, fax at 14.4kbps
- Supports caller ID, wake on ring, telephone answering (V.253), modem on hold
- V.92 software support
Image not in archive — Apple USB Modem