We have recently added the following interest related calculators (with formulas, explanation, and sample problems):
Continuous Compound Interest
Effective Interest Rate
Net Present Value
We have recently added the following capital budgeting related calculators (with formulas, explanation, and sample problems):
Net Present Value (NPV)
NPV Excel and Google Doc
Present Value Calculator
Present Value of Annuity
Understanding the Cost of Credit
This section of the site is still a work in progress. More will be coming soon!
For now, some additional resources and calculators are as follows:
How to Calculate APY offering a calculator, formula, and example.