This page demonstrates the SPServices function SPCascadeDropdowns.
The form to the left is simply the NewForm for the Sales Opportunities list copied into this page for demo purposes. The form itself isn't customized; it's just the standard List Form Web Part (LFWP).
There are two "cascades" applied to the form using SPCascadeDropdowns: Region to State and State to City. The Relationships between the columns are contained in the States and Cities lists, which you can access from the Quick Launch if you'd like to take a look.
When you select a Region, the available States are filtered; when you choose a State, the available Cities are filtered.
- You can see the script which makes this happen here.
Note that I am also calling the following functions to make the form easier to use: