Commercial software no longer available for purchase, usually older titles.
Abandonware programs are frequently from companies that have gone out of business, programs which are no longer under development, or from individuals who have stopped distributing them. They are often only available by searching for them on the internet. This is a grey area in the concept of piracy, much as REAL discarded items are considered legally in the public domain.
Open Source vs. Abandonware
One of the benefits to GPL‘d software is, if the project is abandoned, that the it can always be picked up by another contributor or forked. GPL software is therefore almost never considered Abandonware. Many other Open Source licenses also behave this way but the GPL license was crafted to encourage fewer programs to become abandonware, by forcing derrivative programs to release their source code.
From Abandonware to Free Software
Sometimes, a program from a individual/company which is no longer being developed or which has become flat-broke is released as Free Software. This has happened with Blender (http://www.blender.org). The company behind Blender wanted $200.000. After a few months, the amount (and even more) was donated by various people via the internet. The stories also hit Slashdot. After the source was released under the terms of the GPL, a Blender conference (http://www.blender.org/modules/bc2002/index.html) was in held 2002 in Waag Society, to celebrate.
TakeDown.NET -> “Abandonware”