valve_head2 Today Valve Software officially announced that its Steam digital game distribution platform will be coming to the Macintosh in April. But they are not stopping there. Macintosh owners who have already purchased the PC version of compatible Valve games (those built on the Source engine, such as Half-Life 2, Portal, and Left 4 Dead) will get the Macintosh version free. (And this will continue into the future, too: buying a new game will get you both versions from now on.) Imagine if buying the Kindle version of an e-book bought you the eReader version, too for one price—and...