See Reply #251 in this thread for more examples...
*Edit*
I think I like this favicon better:
The big S looks good. I hacked a quick tiny 16x16 image:
And a quick 32x32 (It would be nice to have a favicon in a few resolutions, so people with high DPI screens can get a higher res logo):
(note: these seriously were quickly hacked together, and spending some more time on it would actually help it to look proper)
I think a custom made low-res SFML looks good (like the 16x16 one here). If the low-res version aren't custom made than you get weird blurring/resampling when downsizing.