Is anyone aware of a free downloadable CAD library of standard parts/ fasteners for Solidworks? We have been working to create our own as we find parts we use regularly, but by no way is it complete. If you know of any links to free CAD libraries please post here. It would a time saving effort to have files of things like: 10-32 skt head cap screws, 80/20 hardware, pneumatic actuators, gearboxes, sprockets, pulleys etc. available locally without having to go to the individual manufacturers website each and every time we need to use something. Understanding these libraries are big, file storage is cheap these days.
I think you should check out the solidworks toolbox. It is built into the program already and has a huge library of things like fasteners, gears, etc.
I have a little collection but I don’t think its what you want.
I know of two,
Bit Buckets posted their’s a little while ago.
Bit Buckets CAD Library
Spectrum has been making one for a few years and we upgraded it a lot this summer/fall with still a bit more work to do before the season.
Spectrum Library
Both of these include a lot of the same parts and sources, like Vex Pro, AndyMark, Team 610’s Part Library, McMaster-Carr, Andrew Lawrence’s gear library and various vendor websites.
Making a CAD Library has been on my to-do list for a while now. Thanks for asking my question for me! 
But even if I download one of these libraries, I’ll want to rename and move and edit everything to make sense to me… maybe I’ll be better off making my own. I’ve never seen Solidworks mate references in these CAD Libraries.
Btw, if you use Solidworks you can make a part template of common extrusions (2x1, 1/2’’ hex, etc.) and start a part with the template, saving the precious seconds it takes to sketch 2 rectangles or 1 hexagon…
When it comes to fasteners we have used a combination of sources:
Vex
McMaster-Carr
Solidworks toolbox
We like Vex 8-32’s line a lot… as the heads of these screws have a lower profile and cover well… McMaster-Carr can go beyond the scope offering a wider range (I like their sidewalk bolts for even lower profile bolts). Finally there is the solidworks toolbox… I use this for screws I buy at Lowes (or home depot). I’ve also used this for the M3’s from Andy Mark for the NeveRest motors.
Finally for parts that we can’t find in these places we make ourselves via a caliper and using this table for diameter and threading.
While it would be nice to have a universal library… I like how the vendors (e.g. listed above) provide the files with a part number so that when it comes time to order the parts… it makes it a bit easier to locate where to order them… we have a folder structure called ‘common’ and this contains a list of folders based on Vendors and from these the parts follow. It’s not a perfect system, but so far it is pretty good (and is certainly open for improvement).
3D content Central is a helpful sight toward that end. TRACEPARTS is another but I have yet to use it.