Although there are expensive, specialized ring-flash strobes from the major lighting manufacturers, again, Paul C. Buff offers the most value-packed product for the money in this product category with his ABR800 ringflash, at only $399 (and, of course, Profoto, Broncolor and others have their versions for a lot more).
ExpoImaging’s RayFlash attachment for Speedlights.
If that’s still too steep, ExpoImaging’s RayFlash Speedlight modifier also produces that signature ring-flash look (“shadowless” lighting, circular catchlight, etc.) for only about $140. It attaches to most on-camera Speedlights, and uses fiber-optic cables to channel your own Speedlight’s output into a ring-shaped light source. I have the original model for my Nikon SB-800, and it works as advertised. There is some light-loss, though I don’t recall exactly how much (probably 1-2 stops).
Here’s some images using a RayFlash Speedlight modifier on a Nikon SB-800 as the only source: