Couldn't live without? That's pretty strong... But I suppose (unless you're counting Linux itself) my answer would have to be the gcc toolset. But if it vanish

Couldn't live without? That's pretty strong... But I suppose (unless you're counting Linux itself) my answer would have to be the gcc toolset. But if it vanished tomorrow, I'd figure out an alternative, so it's not really a "couldn't live without" thing.