Anyone know of any good techniques / tools for cleaning up compression artifacts in jpgs? In particular the ghosting next to sharp edges. Thanks, T.

Simplest way: DON`T use Jpeg ;)

There is no easy way besides cleaning up pixel by pixel. Jpeg is a lossy compression format. And creates artifacts by this. Lost informations cannot be restored. And that's why you better avoid Jpeg when you need a clean chroma keying.

Yesterday I stumbled upon a harder way: using vector quantization to replace degraded JPEG blocks one by one with similar healthy blocks from a codebook. The system is meant for comics and line art.
Paper and Windows demo by Tien-Tsin Wong.