Same way that you do it now - copy the relevant files out of ~/Steam/steamapps/ to wherever you need. I've gotten around re-downloading huge games because everything you need is in there (when I've already done it once before on a different machine, or when I'm getting a game a local friend of mine already downloaded). The only thing stopping you from playing it is the DRM on the executable, but the mythical Steam-is-dead-here-you-go-patch will solve that.
Torrents. And I bet it would be legal because Valve would do something like releasing the games for free just before they tanked. I'm not so sure about some of the other labels that exist on Steam, though.
