I like the way it looks if the input image has some transparency.
Like this:
I'm not going to use such an avatar, mainly because I prefer if avatars are <20 kB since not everyone has broadband connection, but yeah, the effect is quite nice :]
Gratz EmeraldTiger.
By the way, if someone wants to avoid the ugly vertical line on their spheres, there are a number of solutions. One is to use the seamless filter in GIMP. You'll have to experiment with that to find a way to use it without destroying the image too much.
Another way is to add a few pixels on the right edge of your image and fill it with some gradient.
Another way is to duplicate your image (two copies side by side in one layer), then use the blur tool on the edge, then use that blurred 'edge' as the center of your image, and cut both copies so that the original centerline of both pictures is on the edges of the final image.
There are probably other ways, and experimenting is the best way to find them.
Google is second