Found this webiste which has a very good listing of companies that either sell or manufacture ergonomic keyboards. A couple ergo keyboards to add to the list:

• Adesso Tru-Form Media (or Pro)
• Various keyboards made by ComfortKey
• The FrogPad (one-handed keyboard)

Probably the most interesting out of these is the Adesso, since it has a fairly standard layout, USB, and a split spacebar. Adesso also has some Mac specific keyboards: A mac version of the Tru-Form, as well as the Intellimedia pro which looks surprisingly like one of the older MS Natural keyboards. But it has USB, and a split spacebar, as well as the various Apple-specific keys.