When Second Life was first launched in 2003, its client application only supported Windows rigs. Recently, Second Life now supports Mac OS X 10.4.3, which shouts some what of the ever increasing Mac installbase. Other platforms supported include Linux.
Second Life, is a 3D virtual realm and unlike common online games, its existence serves to emulate real life and provide a truly 'global village', where people from all over can meet and get to know another in a 3D rendition of real life. More than that, Second Life also includes the added dimension of commerce, where some real people have downed their entire life savings to invest in virtual real estate within Second Life. Collecting rent and carrying out commerce.

You'll need to download the Mac OS X Second Life client, register at the Second Life site, and you're on your way to your virtual existence. Signing up entitles you to join, the over 7 million other registered Second Life citizens.
Still staring blankly? - "what is Second Life?"