Zeal is a simple offline API documentation browser inspired
by Dash (OS X app), available
for Linux and Windows.
Zeal is released under the GPL license, which allows anyone to contribute
changes. And it is available free of charge.
All documentation available for Dash is also available for Zeal.
See list on Dash's website.
You can also generate your own docsets using the same format Dash uses - see
instructions on Dash's website.