For quick-and-dirty purposes, GIMP is great. Unfortunately it's limited to 8-bit grays, which isn't really enough for critical editing---but if your editing in the digital domain is critical, it's off-topic for APUG anyway. :-)

In the Mac world, at least, there doesn't seem to be a free alternative with 16 bits per channel. GIMP keeps getting closer but isn't there yet.

-NT