GameTap is an online gaming service provided by TBS with a huge range of games available for only $60 a year. That fee gets you unlimited access to over 1000 games of many different varieties.
Essentially, the program is a download service - once you buy your subscription and download the Gametap program, you're shown a list of downloadable games (with the option to sort them or make a playlist of your favorites). The games are both new and old, with new content being added frequently. Among Gametap's licensees are Konami (Contra, Castlevania), SNK (Samurai Shodown, Last Blade, King of Fighters, Metal Slug), Capcom (Street Fighter), Ubisoft (Splinter Cell, Ghost Recon), Eidos Interactive (Tomb Raider, Hitman), Sega (Sonic the Hedgehog), and Interplay (Baldur's Gate, Planescape: Torment). Other games of note include Jagged Alliance 2, Homeworld, Zork, Space Quest, Silent Hill, Psychonauts, and Age of Wonders. So, as you can see, there is a huge variety of games available using GameTap, either if you want to try relatively new games or relive old ones. Furthermore, these games are from many different systems, not just Windows, so you get multi-platform functionality as well.
As far as more technical stuff goes, I was able to run most of the games pretty well on Vista (which I doubt would be the case if I just went and tried the CDs directly), but the success rate of using individual games tended to vary. Games not on a Windows platform (mainly Neo Geo and arcade games) tend to run as a GameTap window, which means they usually work very well, while Windows games actually try to run themselves like regular Windows games. This makes the Windows games vaguely unstable; for example, while Homeworld ran fine with no problems, Homeworld 2 gave me emulation errors despite (or possibly because of) being newer.
While GameTap's staff is available to provide technical support, the issue really lies more with the age of the games than with the program itself. But for the most part, with the investment that it takes, GameTap definitely is a good program for anyone with a huge amount of free time to fill.