The other freebie to get is netfabb Basic. You'll need it to fix your own creations (if you do any) and a lot of the things you'll download off of places like Thingiverse need repairs to print properly. I personally also use 123D Design (also a freebie) to make minor modifications to STL files. It won't directly edit them, but it will treat the entire STL as a solid that you can then add things to, drill holes in, cut in half, or whatever else you find a need to do. You can also use it to do your own supports / braces if you decide that would be your best option.