Cygnus Systems of Michigan patented thumbnail icons and is suing Microsoft, Apple, and Google. From Ars:
The patent in question is US 7,346,850, called "System and method for iconic software environment management." Its abstract describes "a method and system for storing, navigating, and accessing files within an operating system through the use of a graphical thumbnail representing the video display of the active document within the active application." In other words, Cygnus' patent describes features similar to those of Windows Explorer and Apple's Finder.
Cygnus Systems filed for its patent in 2001, which was awarded in March 2008.
Macworld's commenters are unearthing the abundant prior art. You know, just in case Microsoft and Apple would rather spend years defending themselves in court at unimaginable expense, rather than just pay the troll off.
Microsoft, Apple, Google sued over icon software patent [Ars Technica]