IP Independent Professional. Someone who works for him- or herself in a professional capacity, and has clients or customers but no boss. Typically means a solo worker, although IPs may temporarily join work teams. IPs may work either off-site (from a home office or a separate small office) or on-site (at the client's location).
1099er Synonym for IP. Term originally based on the U.S. "miscellaneous income" tax form which shows how much money clients have paid the IP. The general public would find the allusion obscure: they associate 1099s with minor amounts of money earned from things like bank interest. Only IPs associate the 1099 with their main source of income, and so attach a different (very positive) meaning to it.
consultant One meaning of consultant is the same as IP, but this term is more likely to be used in management- or business-related fields, less likely to be used in creative fields. Another meaning of consultant is "someone who gives advice," especially to businesses; in that case the consultant may not be an IP at all, but may work for a consulting company.
contractor Term often used by information-technology and computer-programming IPs, but also by others. Emphasizes project-based nature of work, and that IPs are free to negotiate the conditions under which they work.
free agent Synonym for IP. Derived from sports. At this time, a typical IP is somewhat unlikely to refer to him- or herself as a "free agent."
freelancer Synonym for IP. In the middle ages, a "free lance" was a soldier who sold his services, rather than being permanently employed by one army. Today, the term is widely used in creative fields such as graphic arts, film, writing, etc.
self-employed Term that's typically used by the IRS (U.S. tax authority). IPs often refer to themselves this way, because its meaning is clear to the general public; but there's a subtle negative connotation to this term (as though the "normal" meaning of "employed" is that you work for a company -- which was actually untrue for most of history).
solopreneur Term used for IPs, to distinguish them from entrepreneurs but at the same time remind us of the similarities. Some people use the terms IP and entrepreneur interchangeably, but they have different connotations. An entrepreneur builds a company, with its own brand that could, presumably, exist even if the entrepreneur left the company (sold it, died, etc.). By contrast, an IP builds a personal brand, tied to the individual IP. In the purest sense of the term, an IP is not necessarily interested in expanding into a company, although some IPs eventually do make this shift.