SweetShark said:
Simple question, but is it safe to play again games from your library or buy something in your account?
From the limited amount of information given out so far: it SHOULD be fine to do anything again now.
From what I've read what happened is basically a caching issue that Valve themselves caused through gross negligence and incompetence. After they shut everything down it was an easy fix and there should be no further problems. Which absolutely doesn't excuse that it happened but at least things should be safe now.
Of course that depends how far you want to believe Valve's claims that they fixed it. But at the very least the outcry seems to have certainly calmed down which might indicate its not broken anymore and personally I've had no problems with Steam myself today thus far.