This year I'm trying to get a head start on taxes, so I've already updated my traditional Numbers template. This is an unofficial, potentially incorrect, use-at-your-own-risk spreadsheet for unofficial, potentially incorrect etc. tax preparation number crunching. I use it so that I can gradually prepare my return as I collect all the correct numbers, as well as to get some quick estimates to answer questions as I prepare (e.g. is this deduction form worth the time it will take to fill out?).
This spreadsheet is only optimized for basic personal/sole proprietor/small business taxes, but within the basic 1040 form it's rigged up to automate just about as much as I could. It was fun many years ago to turn the form's IRS instruction tables/logic into its raw algorithmic form, and now it's fun to not need to sign up for some spammy scammy "file your taxes online for free(asterisk asterisk asterisk)" e-File thing. When I've got everything figured out in the spreadsheet, I then copy it to a the IRS's PDF forms and print on a stack of good old paper for mailing.
You can try it out by downloading my f1040-2012 Numbers template file — it should open as an "Untitled" doc you can save to a new spreadsheet. Would love to hear if you find it useful!
Update: posted a revision which adds a small Schedule A table, half of self-employment tax deduction, and slight update to the mileage rate.