IBM has made a dramatic gesture to the open-source movement, making 500 of its software patents available for free use.

The patents will be available for use to any individual or company working on or using software that meets the Open Source Initiative's (OSI) current and future definitions of what constitutes open source software. The pledge is the largest of its kind ever made, according to IBM, and is designed to spur further innovation.

The OSI definition for open-source software is one whose source code is freely redistributed and modified by programmers, and requires that those modifications are also freely distributed. Common examples include Linux and Apache.

IBM will not assert rights over the 500 named patents however it has also reserved the right to withdraw the pledge and assert them against any party filing a lawsuit asserting patents or other intellectual property rights against open source software.

A full list of the 500 patents involved is available on IBM's website at [pdf]. Among them are patent 5,185,861, registered in 1993, which covers technology that helps microprocessors use their memory caches efficiently; and 5,617,568, registered in 1997, for allowing non-Windows based systems to act as file servers for Windows-based clients.

IBM registered 3,248 patents with the US Patent and Trademark Office (USPTO) last year - the highest of any company in 2004. It has also filed the largest number of patents of any company ever with the USPTO.