US Patent 4,558,302 expires today. The patent describes the LZW compression algorithm used in Graphics Interchange Format (.GIF) files. Other formats such as Adobe PDF, TIFF, and early ZIP implementations use the LZW compression algorithms.
When CompuServe.com introduced GIF in 1987, it presented the format as a free and open standard. By 1994, after GIF had become universally accepted, Unisys and CompuServe announced that due to a mutual oversight, no one had announced that GIFs in fact contained a patent-protected algorithm, a Unisys-owned data-compression technique called LZW — and thus required licensing. Since 1995 Unisys has authorized more than 2,000 LZW agreements, making it perhaps the most widely licensed patent in history. Authors writing programs that output GIF images were subject to licensing fees. With the licensing fees lifted some of the momentum towards the use of the PNG format may evaporate.
Confused about the diffences between JPEG, GIF and PNG formats? Webopedia has the answers.
Hat tip: Slashdot
Update: kuro5hin.org has the story of the Unisys GIF tax.