Link fields with external databases

When we create form fields with dropdowns, I'd like to be able to link those dropdowns with a data source instead of having to manage the dropdown fields manually.

For instance, I maintain a list of disciplines in SharePoint. Instead of managing another list of "disciplines" in forms, I want forms to query the list of disciplines and populate using that field.

In a similar vein, I manage requirements in DOORS. I want a dropdown field that queries the DOORS database for the requirements I need - especially if I could use cascading lists to filter based on discipline, work package, system etc.