The GNU Library or "Lesser" General Public License (LGPL version 2.1)
OSI compliance
With a strict view on the OpenSource definition, the LGPL would be a non-free license. In LGPL section 12, it is permitted to add a clause that could restrict the LGPL to give it's permission only to specific groups, but this would be in conflict wth section 5 of the OpenSource definition. Fortunately, there is no actual software that makes use of LGPL section 12, so all currently existing LGPLd software is compliant to the OpenSource definition.
License type
The LGPL is a reciprocal license with a strong Copyleft
Patents
The LGPL gives patent grants but does it contain rules that help to defend against patent trolls.
The LGPL in collaborative works
The LGPL prevents code flow from code under LGPL into other works being under different OSI compliant licenses. The LGPL however allows to non-LGPL works to link against a LGPL work.
As the LGPL does not allow code merging the OSSCC does not recommend to use the LGPL for new projects.
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