For the most part, gaming classics can be found on sites like www.The-Underdogs.info, which is dedicated to "abandonware" -- old properties that have had their copyrights abandoned by their developers and publishers, now available for free download.
But most classic titles aren't abandonware. They're simply not available any longer, as the price to distribute them is too high to justify burning them onto discs and getting them out on shelves. Increasingly, companies have taken to making them available for download. Microsoft and Nintendo are two notable examples of this trend, offering classic titles for a small fee on their respective consoles, the Xbox 360 and the Wii.
Another notable player in the classic gaming space is Gametap, owned by Turner, which offers a catalogue of titles, recent and classic, available for subscribers to play online. Recently, however, Gametap announced a shift in its business model -- offering a fraction of its catalogue (30-60 games, according to the company) for free, running ads in the game windows as the users play.
As casual games companies like PopCap (which began testing ad-supported games late last year) and others are beginning to discover, the free, ad-supported model of gaming is very viable. I predict it will be especially workable for classic games. These titles already have brand names and a built-in following. Publishers can make them available at a relatively low cost online, and players who don't want to pay for an 8-bit title can happily kill a few hours on ad-funded entertainment.
There's no reason why every classic title can't or shouldn't be made available, whether through services like Gametap, or through developers' and publishers' own properties. As Microsoft and Nintendo's success in selling classic titles through Xbox Live Arcade and Wii's virtual console demonstrates, there's a clear demand -- and nothing stokes demand like a price point of zero.