Here are links to some useful tutorials and examples hosted on other websites, including Xilinx:
Xilinx
EDK Concepts, Tools, and Techniques
A Hands-on Guide to Effective Embedded System Design
Xilinx
Creating an OPB IPIF-based IP and Using it in EDK
Adding Custom Logic to an Embedded Design
Xilinx
Using the RocketIO Controller in Virtex-II Pro for Frequency Multiplication
An example design similar to the Oscillator example on this website
Digilent
Using the Base System Builder
A Quick Start Guide to the BSB
