It's tough to find a good adjustable rate mortgage calculator. They usually just assume rates will continue to rise over time, but in real the real world rates rise and fall.
Wolfram Alpha has taken a different approach. Built by mathematicians trying to harvest useful information from online sources, their adjustable rate mortgage calulator seems more powerful than the first few calulators in Google search results.
You can view an overview of Wolfram Alpha on TED. They scour the web for information - primarily numeric information - and package it in a way that helps you get answers. We know that mathematical models don't always work well (remember the subprime mortgage crisis?), but more information often helps you make an educated decision.
If you want to run what-if scenarios, try Wolfram Alpha's calculators.
Further reading:
It is amazing how one cn manipulate the APR. I myself am excited about the changes going on REG Z and Truth-In-Lending disclosures.