I've just recently discovered Good Old Games, i.e. gog.com. The site specializes in DRM-free, classic DOS-based abandonware games i.e. PC games of the 90's and earlier.
Most titles require Windows XP or higher but a substantial number of titles are also available for OS X 10.6 or higher. I found my childhood favorite point-and-click adventure game, King's Quest VI, bundled with King's Quest IV and King's Quest V for $9.99 on gog.com. Steam Powered has the entire King's Quest Collection from I - VI available for $19.99. Since I am not interested in King's Quest I - III or VII, this is a better deal for me.
Unfortunately, the King's Quest IV-VI package only works on Windows XP or higher, but fortunately, I have Vista installed as a virtual desktop which I could access through Parallel Desktop:
I remember being fully engrossed with King's Quest VI on my first computer, an IBM PS/1, back in 1993. It was a great game then and truly a classic now.
Other classics DOS games, such as Earthworm Jim, Space Quest, Leisure Suite Larry, System Shock 2 and over 580 games are available for reasonable prices.
If you enjoy classic PC games of the past, give gog.com a visit!