Periodically, I get into a conversation with someone about 3d printing and end up rifling off a list of cool 3d apps and resources I've stumbled across. So, it's about time I made a list. If you know of other great resources or information, please enter it in the comments.
Tinker CAD; Simple, browser based 3d modeling software. You can also search and build on others' creations. Free
Fusion 360; Robust 3d modeling software for pc or mac. Free for hobbyists and start ups (earning less than $100k)
123 App; This is a suite of multiple apps and software for 3d design. 123D catch allows you to generate models from photos and 123D design is basic design. There are some changes coming to these apps soon. From the press release, it looks like they're consolidating apps and rolling some features into other apps. Be sure to read the comments as well where there are further clarifications.
Cookie Caster; Draw your own cookie cutter or make one from a photo. You can then download the file to 3d print it.
Thingverse; A platform for sharing designs. You can download and build on other designs or just download and print.
Instructables; User generated instructions on everything from construction to sewing, cooking and electronics. 3d printing Instructables may have also downloadable designs. At minimum, a great place for inspiration and learning. If you're over there, be sure to look me up.
YouTube; User generated video. It's a great place to see these apps in motion. You can often find a quick demonstration of exactly what you need. Be sure to check out my Maker Freak channel when you're there (though I have no 3d printing videos yet).
Fabricated; Book on how 3d printing will change the world. I had to stop reading it before bed because it would make my mind race with the possibilities. (Link is to Better World Books who sell books for non profit agencies and use their profits to support world literacy. Not to mention great prices and free shipping).
MIT app inventor; Great resource to start learning about design and building of apps. Companion app (to test your code) is Android only.
Scratch; Basic drag and drop programming
Disclosure; A number of the resources I'm recommending are from Autodesk Inc., but that's just because they're awesome and offer a bunch of free resources in support of makers and hobbyists. My brother works for Autodesk, but I have no relationship with the company nor have received any compensation for my recommendations. (But hey, Autodesk, if you want to throw some $$$ my way, I'd be happy to delete part of that last sentence).