USnooBie is completely open source, here are all the schematics, board design, bootloader, and other relevent stuff for download.
Board Layout: .brd file (20101019) for CadSoft Eagle. Please note: This .brd file's netlist does not match the .sch file, this is because the matching .sch file is very messy and I don't want to show it, so I made a neat looking .sch file instead.
Bootloader (includes source code, precompiled .hex files, and fuse bits to be used): .zip file (20110116)
Objective Development's V-USB: Visit the V-USB download page
USBasp Drivers: Visit the USBasp website. You might need this to use the USBaspLoader bootloader.
WinAVR: Visit the WinAVR website, a more up-to-date but less well-known alternative is the MHV AVR Tools, Atmel releases "AVR Toolchain Installer" along with their AVR Studio, Mac users can check out Objective Development's CrossPack, Linux users can install AVR-GCC and other AVR tools through the package manager.
ATmega328P, datasheets and documents from Atmel's website.
V-USB Documentation Wiki, the driver API is a very important guide.
USB in a Nutshell is very good at helping you understand USB. (This appears to be an older version of the same site)
libusb is an open source library allows people to write USB drivers and applications. libusb-win32 is libusb with tools used specifically for Windows. LibUsbDotNet is a .Net wrapper for libusb which allows you to write .NET applications using libusb.
Driver Signature Enforcement Overrider, you may need this if you are using 64-bit Windows 7 or Vista. Use it to sign drivers you write, and to put Windows into "Test Mode" to allow you to use unsigned drivers.
General AVR Microcontroller Development Tools
AVR Project IDE, this is a development environment I wrote. It is made by myself for myself. You might need it to open projects I write. It acts as an replacement to AVR Studio.
MHV AVR Tools is an updated replacement for WinAVR.
WinAVR, now it is outdated, but it was very popular, and was usually utilized along with AVR Studio (before Atmel started providing the "AVR Toolchain Installer" themselves).