Counter-Strike games are available to download via Steam.
Steam
Yes it is. You need the Steam application to download it.
You have 4 options: 1) If you have the game on Steam in Windows, chances are that the Mac version of Steam has a Mac version of the game available and you may download it for free. 2) install Windows with the Mac OS X app "Boot camp" and play it directly on Windows on your Mac. 3) Download CrossOver (www.codeweavers.com) 4) Try a virtualized solution (Paralells or Fusion)
Dont use a Mac
No, because you are not using Steam to play the game, you are only using Steam to verify the game and to use the multiplayer parts of it. If you have a Mac version DVD/CD of the same game then you could still use it on steam if you register it on Steam.I THINK if you bought the Windows game on steam they will let you download the Mac version on the same machine. They have the full instructions on their web site.
To download custom flat rides for RollerCoaster Tycoon 3 on a Mac, first ensure you have the latest version of the game and any necessary patches. Download the custom ride files from a reputable source, usually in a .zip or .rar format. Extract the files and place them in the "Style" or "Object" folders within the RCT3 directory, typically found in your Applications folder. To prevent crashes, make sure the custom rides are compatible with your game version and avoid downloading too many at once.
I know for sure that TF2 works for mac, but I'm pretty sure you need steam for Gary's Mod, and steam is only for PC. Although, if you really want it you could install Windows on your mac with "Bootcamp", or download steam with "Cross-Over Games" which are both available for download at the Apple website. Just so you know, you need an Intel based Mac to install "Bootcamp" and "Cross-Over Games"... Hope that helps.
You can get classic Mac games from Steam. They have many games for the Mac, Classic and New to download direct to your Mac. Some are Dragons Lair and Max Payne 3.
Steam will be available for the Mac sometime in April (See links below).
I don't think you can for a Mac. I think it's only available for other types of computers. If you do have another type of computer, you have to download Steam and pay for the game.
To play RCT on a Mac, you will need to buy RCT3 platinum version on retail, online or mac app store.