SuperLoanCalculators.com often reviews websites with online loan calculators and even rates them according to criteria such as whether the website requires registration, whether special software or plugins are required, and the quality of the loan calculator itself.

Some of the online calculators you may find useful include:

BankRate.com - I almost always find myself recommending Bankrate.com. They have many excellent tools, and I'll be writing a complete review of their site in the future. Among other things, they have a very nice suite of loan calculators of all kinds, including of course a simple mortgage calculator, and also a fixed rate versus adjustable rate mortgage calculator. Bankrate.com's fixed rate mortgage calculator, which they simply call a "Mortgage payment calculator," lets you plug in your mortgage amount, the term (the number of years of the loan), and the start date of the mortgage. You click the calculate button, and it tells you the amount of the payment. Very simple. As I mentioned, the fixed rate mortgage is a very simple type of loan to calculate. The calculator also has an "extra payments" section that lets you calculate how much sooner you could pay off the loan if you added a fixed extra amount every month, or if you added an extra yearly payment, or even if you added a one-time extra payment on a particular date. It's a nice tool.

CNNMoney.com has a good mortgage calculator. From the home page, mouse over the Real Estate tab at the top of the page, and select Home Finance Calculator from the pulldown menu. The calculator has certain "assumptions" built in. CNNMoney says about this: "We've assumed a 30-year mortgage term, annual property tax of $3,500 and homeowners insurance of $481 -- the national average. And we do not factor in private mortgage insurance, which you'll owe if your down payment is less than 20 percent of the purchase price. It averages from $50 to $80 per month. Plug in your own numbers for more tailor-made results."

EasyCalculation.com is not a loan calculation site in particular, but it's got a ton of calculators of all kinds, with the links to the calculators conveniently placed right on the front page. In the Mortgage Calculator section, they've got all the following types of calculators: Simple Mortgage (that would be the fixed rate mortgage calculator that this page discusses), Amortization, Loan Comparison, Adjustable Mortgage Payment, Down Payment, Interest Only Mortgage, Mortgage Prepayment, Pre-Payment Penalty, Mortgage Refinancing. You can't fault them for thoroughness! Be aware, though, that the calculators are pretty basic and don't offer all the bells and whistles like the sites listed above.
This is not an exhaustive list of websites with good mortgage calculators, as there are many others available on the net and I haven't attempted to review them all. But the ones mentioned are among the best and should more than meet your needs.

Frank Herbert is an experienced freelance writer specializing in financial topics, and is founder several blogs dealing with subjects such as online bill pay, money transfer services, and mortgage loans.