(From the 1930 Sellar and Yeatman parody "1066 And All That") Often capitalised; always pronounced as if capitalised.
1. Self-evidently wonderful to anyone in a position to notice: "The trailblazer's 19.2 Kbaud pep mode with on-the-fly lempel-ziv compression is a Good Thing for sites relaying netnews".
2. Something that can't possibly have any ill side-effects and may save considerable grief later: "Removing the self-modifying code from that shared library would be a Good Thing".
3. When said of software tools or libraries, as in "yacc is a Good Thing", specifically connotes that the thing has drastically reduced a programmer's work load.
Opposite: bad thing, compare big win.