I have PC games but just got a Mac, can I get free Mac versions of these games?

I just switched to Mac and love it, however, I can't run any of my games on it. I know that there is the Bootcamp thing but I can't find the resource CD from my PC with XP on it and don't care enough to buy a new copy of Windows. So I was wondering if there was a way to get free Mac downloads of the PC games I have? I know that Blizzard offers a free Mac download of Starcraft by going to their site and putting in the CD key. Does anyone know if Sierra or Maxis does this? I've looked and got nothing.