I found this extremely neat little tool today. If you’re a freelancer and have struggled with setting your rates, try this Hourly Rate Calculator.
I tried it and it came up with something fairly close to what I’m actually using, happily. 
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I tried the one at Freelance switch but don’t like the multi-page thing.
There’s another calculator at a site called Skillserv which does everything on one page.
Check out the tool that I’ve created (http://www.xbarit.com/bradblog/index.php/2007/10/12/billable-rate-calculator/).
It’s Excel-based, so it’s downloadable and you can see the formulas involved. I’ve also provided comments throughout that will help folks understand the financial mechanisms involved.