A digital video compressor/decompressor from RAD Game Tools, Inc (http://www.radgametools.com/), developed by Jeff Roberts. Mostly used in DOS, Windows and MacOS-based games.
Back in the day, Smacker was very popular among computer game developers, because the video quality - in 256 color mode - was very good and files were small. According to RAD, the codec was used in over 2000 games; Personally, the first game I saw it used was Warcraft II.
RAD Game Tools offers Smacker compressor/decompressor (RAD Video Tools) freely - they do, however, charge for the SDK. These days, it's really good for converting All Those Golden Old Game Intro Videos to newer, standardized open formats. =)
Smacker has since been superceded by Bink, the truecolor-mode compressor by same company, but there are other popular codecs for truecolor video these days (MPEG being among the most popular these days) so there's no single gigantic player on the field anymore.