Auto calculating sheets do the heavy lifting for you by helping you build characters. They typically implement some or all of the character building rules.
Excel Character Builder/Sheet by Memetix
Hands down the best tool for building a character. It handles Dark Heresy, Rogue Trader, Deathwatch, and Grey Knight characters, and supports all rulebooks. If you are working on your character any other way you are working too hard. This will handle all calculations and options for you for those systems. Memetix is a great guy and can be reached on the FFG forums in this thread if you have issues.
Auto-calculating sheets aren't for everyone, here are he traditional printable character sheets that you fill out as you see fit. Generally much better for characters that you will keep track of on paper.