Mobile Developer Tips has written a number of posts about how to define and call Custom URL Schemes:

- Check If Custom URL Scheme Is Defined

- Launching Your iPhone App Via Custom URL Scheme

- Launching iPhone Apps With Custom URL Scheme – Part 1

- Launching iPhone Apps With Custom URL Scheme – Part 2

If you are interested to get a list of defined schemes for third party applications, here are two websites that I am aware of:

- http://handleopenurl.com/scheme

- http://wiki.akosma.com/IPhone_URL_Schemes

Please post a comment if you know of other similar resources.