There are not many online casinos that support Mac OS systems, however usually you can easily identify operators who support Macintosh, as they clearly state it on their homepage.
Note that instead of looking for Mac free online casinos, you can also check for no-download casinos or flash casinos, as they render directly in you browser, eliminating the need to download. The most popular online casino software is Macromedia Flash, which provides a rich and colorful free online gaming experience. The second most popular software is Java by Sun Microsystems. Java isn't quite as nice as Flash but allows for some flexibility with your game play and preferences.