The layman version: a beneficial or positive feeling about something. The hacker version: self-evidently wonderful to anyone in a position to notice. As in, "The Trailblazer's 19.2K baud PEP mode with on-the-fly Lempel-Ziv compression is a good thing for sites relaying netnews."
It may also be something that can't possibly have any ill side-effects and may save considerable grief later, as in, "Removing the self-modifying code from that shared library would be a good thing." When said of software tools or libraries, as in, "YACC is a good thing," it specifically connotes that the thing has drastically reduced a programmer's workload. The phrase came from hackers long before it came from Martha Stewart.