Microchip USB Framework
Microchip provides a USB Framework free for download, which includes MPLAB/C18 projects to get you started with USB programming for PICs. These projects are generally intended to be run on one of Microchip's many demo boards, but most of the code can be taken and used for any USB project. The framework also includes both PIC and PC software for two USB bootloaders.
You can download the framework from Microchip as one part of the "Microchip Applications Libraries," bundled with some other stuff like a "Graphics Library" and a "Memory Disk Drive Library": framework download