This is what I use: http://www.adp.com/tools-and-resources/calculators-and-tools/payroll-calculators/salary-paycheck-calculator.aspx
Works great. I play around with current paycheck settings to make sure it returns the correct number, then adjust my 401k contributions to estimate take home pay.
What's scary is that I can't replicate my ADP paycheck using the ADP calculator! It appears that the way one benefit item is represented on my pay check doesn't line up with how the calculator thinks it should work.
EDIT: I think I finally figured it out. My 401k withholding is set by % of gross pay; however, there is an additional employer-paid benefit that's taxable, so they add that amount to the gross pay on my statement, and then subtract it again later on. The problem is that this is not factored into the 401k %, so when I try to fix the FICA numbers by including the "phantom" benefit pay, it throws off the 401k amount in the calculator. It all works if I use the actual dollar amount for the 401k withholding, but that defeats the purpose somewhat if the goal is to play what-if with the 401k withholding percentage...