Aggregate related records

Looking for a way to show aggregates (sum) of related (1:N) records on the parent form.

For example, in a 1:N of Project -> Task, where Task has field taskBudget, you could display sum(taskBudget) on the Project form. 

I'd also like to handle multiple levels of 1:N (otherwise a rollup field might suffice). For example Project->Task->Subtask, and show sum(subtaskBudget) on Project form.