to have granted royalty free licenses to hundreds of not-for-profit organizations that used the patented LZW compression method and, by extension, the GIF image format. However, this did not include most software developers and Unisys were "barraged" by negative and "sometimes obscene" emails from software developers. Licensing Patenting software is widespread in the US. As of 2015, approximately 500,000 patents had issued in the 23 classes of patents covering "computer implemented inventions" (see table). Many software companies cross license their patents to each other. These agreements allow each party to practice the other party's patented inventions without the threat of being sued for patent infringement. Microsoft, for example, has agreements with IBM, Sun (now Oracle), SAP, Hewlett-Packard, Siemens, Cisco, Autodesk, and recently Novell. Microsoft cross-licensed its patents with Sun, despite being direct competitors, and with Autodesk even though Autodesk has far fewer patents than Microsoft. The ability to negotiate cross licensing agreements is a major reason that many software companies, including those providing open source software, file patents. As of June 2006, for example, Red Hat had developed a portfolio of 10 issued US patents, 1 issued European patent, 163 pending US patent applications, and 33 pending international PCT (Patent Cooperation Treaty) patent applications. Red Hat uses this portfolio to cross license with proprietary software companies so that they can preserve their freedom to operate. Other patent holders are in the business of inventing new "computer implemented inventions" and then commercializing the inventions by licensing the patents to other companies that manufacture the inventions. Walker Digital, for example, has generated a large patent portfolio from its research efforts, including the basic patent on the Priceline.com reverse auction technology. US universities also fall into this class of patent owners. They collectively generate about $1.4 billion per year through licensing the inventions they develop to both established and start up companies in all fields of technology, including software. Still other patent holders focus on obtaining patents from original inventors and licensing them to companies that have introduced commercial products into the marketplace after the patents were filed. Some of these patent holders, such as Intellectual Ventures, are privately held companies financed by large corporations such as Apple, Microsoft, Intel, Google, etc. Others, such as Acacia Technologies, are publicly traded companies with institutional investors being the primary shareholders. The practice of acquiring patents merely to license them is controversial in the software industry.