Ive heard that Raynox are the ones you want.
I have a Raynox 2.2x converter for use on my FZ20, they are better than the other makes I have seen, except for the original Panasonic one which is far more expensive. The only issue is that you have to zoom in almost completely to avoid vignetting at the corners. At wide angle you can see a complete circle. OTOH it does give me a 950mm equivalent lens for relatively little money.
In order to use a wide angle you have to zoom in so that the angles work properly, otherwise you would get a severe vignetting problem. This can lead to other issues and the use of a fisheye converter is relatively limited anyway so I never bothered to get one.