M-Stack (link) is a new, well-documented, free and open source implementation of a USB stack for Microchip PIC platforms developed by Signal 11. That is Alan Ott’s creation, a name that many of you will immediately associate to the excellent cross platform HIDAPI libraries.
The software is dual-licensed under the LGPL version 3 and the Apache License version 2.0. It may be used without royalty payment in both commercial and open hardware projects so long as the conditions of either the LGPL or the Apache License can be met.
(Commercial licenses are available for purchase for companies and projects which cannot or wish to not comply with the terms of either the LGPL or the Apache License.)