I don't think there are any registered bits*. Take a look at threads by @alexpikkert (like this one, this one, or this one), @Philip_Verbeek (most of his threads pertain), @drthuler, @StuckInSynth, @codewizard58, a bunch of others - especially @JackANDJude, who started me down this amazing rabbit hole. There are a bunch of very adept hardware hackers here, and just about everybody I have run into is more than willing to help!
In the spirit of open source design, discussion here leads to greater understanding of the use and design of the custom bits. So tell us about your ideas, and what bits you think are needed. Once the bitLab reopens, you will be able to download the littleBits design guidelines, but in the meantime, study the schematics of the existing bits for clues on how they work.
* I had a bit in the bitLab when it went on hiatus. I was told that I would need to resubmit it when the 'Lab reopened. I think they will start it with a clean slate.