What is fRaBs and how does it relate to Abuse?
Parts of Abuse were, after the game's profitability had waned, released to the public domain by Crack Dot Com. This did not encompass the totality of the game's data; music, sounds and the full version levels were excluded, but it did include the source code and shareware version data. Crack Dot Com also reserved the trademark to the title "Abuse" as they, at the time, still planned to create Abuse 2 (which never happened).
This spawned a fan effort to create a "Free Abuse" or "fRaBs" as it came to be called to avoid trademark issues. The four shareware levels and the other game data were expanded upon with a collection of fan-created levels and mods arranged into several episodes connected by a hub map. Together with various ports made possible by the source code release this created an ostensibly free Abuse game. However, this project is probably best considered "free as in beer" and not proper FLOSS as some material included (such as the sounds and music which were never properly replaced, and possibly some of the user-created mods) might be questionably licensed.
IMO, fRaBs is not the best way to experience Abuse for the first time—the full commercial release of Abuse was much more polished compared to the sometimes dodgy quality and difficulty progression of the fRaBs material. However, I do definitely recommend it for any Abuse fan who wants more, as some of the levels and other data appear to survive only in this project, not as the individual Abuse mods they may have started out as.
A page for fRaBs is still online at abuse.zoy.org although it appears not to have been updated for many years.
DOS fRaBs releases
Here is a DOS release of fRaBs. It might be outdated; later development seemed to be done mainly as Linux compile-from-source releases.