You can buy Portal from Steam. Portal was free on many occasions but it is currently a paid product.
It will be on Mac the day Portal 2 is released.
Buy it through steam.
Once in a while, Valve will have portal be free for a while.
Yes it is. You need the Steam application to download it.
PC, PS3, XBOX 360, Mac,
ps3, Xbox 360, PC/MAC
No. It will cost money. Portal costs money. It was only free for a limited time.
The internet
The Mac OS comes free with a Mac. New versions have a small charge.
no pc and mac only
If you want to play the game on a PC or a Mac, yes.
No, they just make it for PS3, XBOX 360 and PC/MAC.