"I am sure a lot of you remember the great 'GIF fiasco': more
than a decade ago, Unisys decided to make money out of the most
used image file format on the Internet: the GIF format. To be more
precise, Unisys announced that they would go after developers of
programs able to load and save GIF files (never mind the fact that
even back then there was plenty of free software which wouldn't
have been able to pay).
"To make the short story shorter, the PNG file was invented as a
reaction to Unisys' move; although it was never wildly successful,
PNG did manage to make Unisys's threat very much irrelevant..."