for anybody not in the know -- ssbumps are a specific shader technique Valve developed for The Orange Box. they're effectively a sort of normal map which contain occlusion data, allowing for materials to appear to cast shadows onto themselves.

they used this extensively in HL2 Episode Two to give the caves section more apparent detail than regular normal maps could achieve without more complex geometry (these tunnels are effectively low poly tubes for the most part)

if anyone's curious, they wrote a short SIGGRAPH paper on it: cdn.fastly.steamstatic.com/apps/valve/2... my own explanation probably doesn't do it much justice!

I think the reason these didn't take off is because graphics tech in games was largely moving in other directions at the time. and these days I don't think they serve much purpose with more modern lighting techniques and levels of fidelity across the board.