] More and more people are becoming aware that the reading and/or
] writing of GIF images requires a license to use Unisys patented
] Lempel Ziv Welch (LZW) data compression and decompression
] technology, including United States Patent No. 4,558,302, Japanese
] Patent Numbers 2,123,602 and 2,610,084, and patents in Canada,
] France, Germany, Italy and the United Kingdom. Since January of
] 1995, Unisys has entered into almost three thousand license
] agreements for use of GIF and other LZW-based technology.

Info on the LZW patent from Unisys