Those aren't lenses... well, in a way. They go in front of the camera's lens and either widen or narrow the field of view, make it focus closer, etc. A more accurate term would be wide/teleconverters or macro diopters.

Anytime you add extra glass in front of the lens, it degrades the image, so those accesory 'lenses' more or less suck. Real interchangeable lenses are the only thing that goes in front of the sensor or film, you take the one that's on there off and put the other on.

It's OK to use those with video cameras because there's not much resolution to begin with (HD is less than 2 megapixels, and mini DV is well under 1), but with still cameras it's not really worth it...