Up until now I have tended to agree with people like Jeremy Thake who warn against the use of jQuery in your SharePoint deployment. **UPDATE** - see comments from Jeremy below clarifying his position on who should be using jQuery.
However, a common requirement from users is to provide cascading drop down boxes in the new and edit item forms which SharePoint just doesn't provide out of the box.
I then came across Marc Anderson's excellent jQuery library for SharePoint which provides a solution for creating cascading drop downs as well as several other very useful functions.
Although this does not address all the issues of using jQuery I think that it helps to overcome some of them.
- The cascade function has already been tested for SharePoint 2010 so I know it won't break when we upgrade.
- It is a library which can be reused across SharePoint with minimal additional script needing to be added to individual pages so it can be maintained and updated centrally.
- It is a Codeplex project that is relatively popular so I can have some confidence that it will continue to be updated and even if it isn't I have a copy of the library that I can update myself if anything ever became broken in future versions of SharePoint.
For more information on the library and the SPCascadeDropdowns functionality take a look here.