Someone recently e-mailed me about the original version of Quest for Glory: So You Want To Be a Hero, one of my favorite games of all time. Unfortunately, like many other games, this falls under the category of "abandonware" - that is, it is no longer available through retail channels (and in this case, we couldn't even find the original version on eBay).

He eventually found a site with this game and a whole slew of other old Sierra games which run fine in Windows XP.

Abandonware is quite interesting from a legal perspective. Unlike trademark law, copyright doesn't have the formal concept of abandonment (that is, where the rightsholder doesn't voluntarily give up his rights) - so technically speaking the rightsholder could sue for infringement.  Practically speaking, many of these companies are no longer around and others are no longer interested in enforcing their exclusive rights.

These sites also act in good faith which might weigh favorably in a fair use analysis. Most of them appear to have a strict policy of not including any games available at retail, even when released as part of a "classics" anthology. Thus, it is clear they are not harming the market for the protected work. In the case where the anthology is eventually released, you could even make the argument that they are helping the market by keeping interest in the game alive.