It's not legal, since the copyright doesn't expire until 95 years after the release, IIRC. You can download it, but you'd be doing a disservice to Irrational Games. You can still purchase SS2 through online sites like Amazon and eBay. Sure, it might not be putting money into IG's pockets, but it least it's legal. The only time it is if it has been released as freeware or made open-source, much like Quake 1-3 has been. I'd expect a release of the IG library on Steam, though, since a lot of companies are going that route (look at id for example).