Your student loan lender’s website will usually offer the best loan repayment calculators corresponding to their repayment plans.
These websites offer generic calculators on repayment, calculating interest and general loan management:
Mapping Your Future http://mappingyourfuture.org/paying/standardcalculator.htm
BankRate.com (loans) http://www.bankrate.com/brm/popcalc2.asp
BankRate.com (credit cards) http://www.bankrate.com/brm/calc/creditcardpay.asp