Ricochet
Ricochet (2000), a futuristic action game that challenges your agility as well as your aim, features one-on-one and team matches played in a variety of futuristic battle arenas.
Ricochet was released as a free mod for Half-Life on on November 1, 2000, and was subsequently included in update 1.1.1.0 of Half-Life. When Valve ported its 
titles to Steam, Ricochet was changed to a paid product, with no major content update. Owners of retail copies of Half-Life that register their Half-Life CD key via Steam have been grandfathered in, receiving a Steam copy of Ricochet for free.
The source code for Ricochet can be found in the Half-Life SDK repository, in the 
ricochet subfolder. A fork updated to properly compile with Visual Studio 2019 Community Edition can be found on GitHub.
Official maps for Ricochet use the prefix rc_.