- Alcatel-Lucent, which won US$1.52 billion in a lawsuit which alleged that Microsoft had infringed its patents on playback of audio files. This ruling was overturned in a higher court.
- Apple Inc. (known as Apple Computer, Inc. at the time), which accused Microsoft in the late 1980s of copying the "look and feel" of the graphical user interface of Apple's operating systems. The courts ruled in favor of Microsoft in 1994. Another suit by Apple accused Microsoft, along with Intel and the San Francisco Canyon Company, in 1995 of knowingly stealing several thousand lines of QuickTime source code in an effort to improve the performance of Video for Windows. After a threat to withdraw support for Office for Mac, this lawsuit was ultimately settled in 1997. Apple agreed to make Internet Explorer the default browser over Netscape, and Microsoft agreed to continue developing Office and other software for the Mac for the next 5 years, purchase $150 million of non-voting Apple stock, and made a quiet payoff estimated to be in the US$500 million-$2 billion range.
- AOL, on behalf of its Netscape division. Netscape (as an independent company) also was involved in the United States v. Microsoft antitrust suit.
- Be Inc., which accused Microsoft of exclusionary and anticompetitive behavior intended to drive Be out of the market. Be even offered to license its Be Operating System (BeOS) for free to any PC vendors who would ship it pre-installed, but the vendors declined due to what Be believes were fears of pricing retaliation from Microsoft: by raising the price of Microsoft Windows for one particular PC vendor, Microsoft could price that vendor's PCs out of the market.
- Bristol Technology Inc., which accused Microsoft illegally withheld Windows source code and used its dominant position with Windows to move into other markets. A ruling later ordered Microsoft to pay $1 Million to Bristol Technologies (see also Windows Interface Source Environment).
- Caldera, Inc., which accused Microsoft of having modified Windows 3.1 so that it would not run on DR DOS 6.0 although there was no technical reason for it not to work. Some claim that Microsoft put encrypted code in five otherwise unrelated Microsoft programs in order to prevent the functioning of DR DOS in pre-releases (beta versions) of Windows 3.1. Microsoft settled out-of-court for an undisclosed sum.
- Opera, which accused Microsoft of intentionally making its MSN service incompatible with the Opera browser on several occasions. Sendo, which accused Microsoft of terminating their partnership so it could steal Sendo's technology to use in Windows Smartphone 2002.
- Spyglass, which licensed its browser to Microsoft in return for a percentage of each sale; Microsoft turned the browser into Internet Explorer and bundled it with Windows, giving it away to gain market share but effectively destroying any chance of Spyglass making money from the deal they had signed with Microsoft; Spyglass sued for deception and won a $8 million settlement.
- Stac Electronics, which accused Microsoft of stealing its data compression code and using it in MS-DOS 6. Microsoft eventually lost the subsequent lawsuit and was ordered by a federal court to pay roughly $120 million in compensation.
- Sun Microsystems, which held Microsoft in violation of contract for including a modified version of Java in Microsoft Windows that provided Windows-specific extensions to Sun's Java language; Microsoft lost this decision in court and were forced to stop shipping their Windows-specific Java Virtual Machine. Microsoft eventually ceased to include any Java Virtual Machine in Windows, and Windows users who require a Java Virtual Machine need to download the software or otherwise acquire a copy from a source other than Microsoft.
- Zhongyi Electronic, which, having licensed two self-designed fonts to Microsoft for use only in Windows 95, filed suit in China in April, 2007, accusing Microsoft of using those fonts in subsequent Windows 98, 2000, XP, 2003 and four other Chinese-language Windows operating systems. Beijing's No. 1 Intermediate People's Court ruled on November 16, 2009 that Microsoft violated the scope of licensing agreements between the two companies. The result of the verdict is that Microsoft has to stop selling Chinese-language versions of the aforementioned operating systems. Microsoft said it will appeal. One of the fonts in question may be SimSun.
Many other smaller companies have filed patent abuse and predatory practice suits against Microsoft.
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