If you operate in the form of a sole proprietorship or a partnership, you're considered "self-employed" and thus eligible to set up a Keogh plan. If your business is incorporated, you're not.
In addition, the following people are also eligible to set up Keoghs:
Doctors. A salaried doctor is not considered to be self-employed, even if the "salary" is paid by a corporation owned by the doctor. If, however, the doctor has income from other sources, the doctor could contribute to a Keogh from those other sources.
Consultants. Generally, consultants who receive fees for their
services are considered to be self-employed. The rules, however, are less clear
for retirees who receive consulting fees exclusively from their former
employers. In fact, the courts are split as to whether such a person is
self-employed, with the most recent cases finding that such a person is
self-employed. Those courts have based their decision on a belief that a person
need only hold herself out as available for work to one person (regardless of
who that person is) to be regarded as self-employed.