Maker and Thingiverse user Tyler Bletsch has developed 3D printable nuts that can be used with virtually any size metal bolt or screw. The 3D printed nut is printed with no threads, so when a metal bolt is screwed into it, the threads are cut directly into it, making it a firm and tight hold. Obviously 3D printed nuts are not suitable for any projects or tools that are load bearing, but they have multiple useful applications for other projects. You can find out more on 3DPrint.com: http://3dprint.com/68289/3d-print-any-nut-you-choose/
Below is a look at Bletsch's designs: