In October, I gave a talk about beginning to design your own projects. I make this careful distinction between projects and patterns, because I consider pattern writing to be a completely separate topic, although it is of course dependent on the design process. If you are past the project-design level and are interested in writing down your patterns, I HIGHLY recommend first reading Kate Atherley's Beginner's Guide to Writing Knitting Patterns cover to cover.
Below are some resources I use personally as references or tools during my design process. I hope you find them useful! If you have any suggestions to addd to the list, please let me know in comments!
Foundations - Shirley Paden
I can't recommend Shirley Paden's work enough:
Shirley Paden also has an accompanying Bluprint class (formerly Craftsy)
Sizing & Measuring
Another great Bluprint resource if you are heading in to creating objects in multiple sizes is Faina Goberstein's class Sizing Knitwear Patterns.
Faina's class materials include detailed size charts for women's sizes. Size charts are an essential tool for design if you don't have your "subject" available to you for measurement. You can access free size charts and "standard" size information at the Craft Yarn Council of America's website. These are a good place to start, however you will likely find you end up preferring more detailed measurements charts, as well as more specific body-measuring guidelines.
Ysolda Teague offers a free measurement resource for knitwear designers that fills in many of the blanks left by the CYCA.
For guidelines on how to measure an actual human for a garment, here is a nice printable PDF from a tailor - notice how they use a shirt as a guide in measuring the cross-back width between the shoulder seams. There are many examples like this on the internet ripe for googling. My only warning: beware any instructions that guide you to measure an arm held straight, unless you like sleeves that will always be slightly too short.
I have a very large collection of stitch dictionaries that I add to whenever possible. Even if they overlap in some portion (I have a LOT of instructions for Feather and Fan), you never know what unique gems they might contain, so I pick them up whenever & wherever I come across them. I prefer dictionaries with charts so that I can see a stitch pattern's construction at a glance, so I'm more likely to purchase stitch dictionaries that include charts. If charts aren't your thing, you should check to make sure the dictionary you're holding includes written instructions as well.
A Google search of knitting chart software will bring up a multitude of options, including websites that will let you create your own charts online for free. Currently I use StitchMastery. Programs like this let you quickly sketch out an edit stitch patterns, and design elements, not to mention create professional-level charts for inclusion in patterns.
You also have the option of using Excel or other spreadsheet programs. Here is one tutorial on how to set up your spreadsheet.
Good ol' pen & paper
Of course you can always use graph paper. You can even download or purchase Knitter's graph paper - wherein the "squares" are not actually square, but rectangles that are slightly wider than they are tall - to better reflect the dimensions of an actual knitted stitch.
These are just some of the resources you can find online to help you take the jump "off pattern." I'd love to hear about any others you come across in your travels, and see any projects you design!