We just opened the project section at Fritzing, which we hope will become a general repository for Arduino-based projects (and also for other microcontrollers).
Please have a look at http://fritzing.org/projects
Currently it just features a few basic examples, but we made it quite open so that all kinds of project descriptions are possible - from technical tips to complex installations.
We hope that you find it useful as a one-stop shop for Arduino project documentation. Please let us know how you like it! We will improve it as we go.
Looking forward to your submissions,
the Fritzing team
If you haven't heard of Fritzing yet: It's an open-source tool we are developing at FH Potsdam, to enable non-engineers to document and share Arduino-based projects, and even create your own pcb.