I have used Taxcaster in the past and found it good for my purposes. You might want to compare w/ others
like this one from Block http://www.hrblock.com/free-tax-tips-calculators/tax-estimator.html
Or your full blown TT return from last yr.
Since it is free, it must be lacking certain features of the real tax software so somehow you need to discover
if those things are important for you. One thing I know that Taxcaster (and probably the Block calculator) lacked in the past was the
interaction of muni bond interest and SS.