Abandonware usually refers to commercial software which is no longer sold or supported by its publisher, or whose publisher has gone out of business. In these cases, there is no legal way to acquire the software except through second-hand sources. There are websites devoted to offering abandonware titles for download. Note, though, that copyright law does not recognize the concept of abandonware, so if you download such software, you are violating the letter of the law. Many commercial IF titles fall under this category.
For more information, see the Wikipedia article.