A must have tool for every SharePoint developer. SP Services is one of the best client side library to overcome the limitations of SharePoint. Be it a Cascading lookup or filtered lookup or client side execution of CAML queries or whatever be it, this would be handy to do your job. Check out the detailed documentation in CodePlex to learn more about the capabilities of SP Services.
If you are into SharePoint client side programing without using SP Services then you are missing something big, that’s for sure.