List of notorious software patentssoftware patents include:
- Unisys acquired a patent on LZW compression, a fundamental part of the widely-used GIF graphics format. Unisys published the compression algorithm, but did not make it clear that it was applying for a patent. CompuServe then developed a graphics format, and, having no reason to choose otherwise, chose the LZW compression algorithm. Unisys waited until the GIF algorithm was widely used, and then demanded royalties for users and developers who until then had had no idea of the issue. It is uncertain if Unisys delayed intentionally or not, but it is clear that many developers were very angry at the Unisys response. This resulted in re-development of a new graphics format, PNG.
- The popular music format MP3 is encumbered by a number of patents, which were only enforced once the format became popular. This resulted in several GNU/Linux systems dropping support for MP3, and in re-development of new audio formats—notably Ogg Vorbis.
- British Telecom sued Prodigy under U.S. Patent No. 4,873,662 claiming that Prodigy infringed its patent on web hyperlinks. However, after costly litigation, a court found for Prodigy, ruling that British Telecom's patent did not actually cover web hyperlinks.  Hyperlinks were first described in 1945 in the landmark paper As We May Think, as well in the widely-known project Xanadu starting in the 1960s.
- Cadtrack's US Patent 4,197,590 covers drawing a cursor on a screen using XOR, which allows removal by XORing it again, thus eliminating the need for backing store.
- Amazon vs. Barnes and Noble: "One click buy"
- EOLAS vs. Microsoft: "browser plugin patent"
- Compton's NewMedia was awarded a submarine patent in 1993, which had been filed five years before, for "A search system [that] uses a multimedia database consisting of text, picture, audio, and animated data." Compton's announced their patent at the height of the excitement over CD-ROM software and claimed this patent covered all multimedia software, and announced a royalty payment schedule. An outcry ensued that this was an attempt to patent something that had been in active use for many years, and the furor was so great that the Patent Office commissioner quickly started an "office action" to re-investigate the claim. It seemed that the PTO had only reviewed existing patents for prior art, and not the wide body of prior art in the field that had not been patented. The patent was voided in 1994.
- Stac Electronics sued Microsoft for patent infringement when Microsoft introduced a data compression scheme into MS-DOS which resembled Stac's Stacker software. Stac was awarded $120 million by a jury in 1994 and Microsoft was ordered to recall versions of MS-DOS with the infringing technology. Subsequently Microsoft and Stac settled the case; Microsoft promised not to appeal, paid Stac $43 million, and purchased $40 million of preferred Stac stock.
- The Wiki system itself might be patented by Family Systems Limited, at least according to some interpretations.
- List of Example patents on the AEL wiki
- Microsoft and Stac Electronics
- Examples by Gordon Irlam
- FFII: Well documented examples
- European Software Patents: Assorted Examples
- European Software Patent Horror Gallery
- Ole Tange's ziki: Patent index with 20.000 european software patents
- European Patent applications from Microsoft in 2004, nearly 2 per day