Earth's Special Forces is a mod for the original Half-Life's engine (not Half-Life 2's Source) that transforms Valve's shooter into a massive arena for Dragonball Z battles.
It's a third-person fighting game with a few different modes (you can team up for deathmatches, etc). Being a mod it's a constant work-in-progress, and comes with a warning that "there is a learning curve so newbies beware", but if that doesn't deter you, the mod can be downloaded below.
Earth's Special Forces mod for Half-Life [ModDB]