I was amazed to not be able to find a simple tax calculator online (i.e. given my income, filing status, and state, how much tax will I pay, what will my net income be, etc.). So I wrote one, plugged figures for 2012 into it, and posted it here. This is a quick hack, but it should work quite nicely on mobile devices. (The last time I needed a calculator in a hurry I implemented this — I actually wrote it to use on a desktop machine, but I added a couple of meta tags so it would work better on my iPhone.)
Update
The reference I had for State income tax rates was hopelessly wrong (most states have a progressive tax system — actually all of them, if you count “no state tax” as progressive). I rejigged the code to use JSON feeds for all input data (except dependent deductions, which I hardwired for now). Because state taxes are also progressive, I refactored the code to use a single function for calculating progressive tax scales against a given figure. I didn’t have the patience to encode every single state tax table, so it only works for a few example states including those I happen to be interested in.