posted February 10, 2008 and revised April 07, 2010
Loanr is a loan calculator for mobile (J2ME) devices. It can calculate monthly payments and total interest given an APR and principal; or it can perform the reverse calculation and calculate the principal (and interest) given an APR and monthly payments.
It is also available via the web, which is useful if you happen to have a modern smartphone and a mobile Internet connection--especially if your phone doesn't happen to support J2ME.
Loanr is free software.
OK, this is working pretty well. Time to upgrade for no reason.