[sdiy] Best Modular DIY projects?

[cheater cheater]

Hey guys,
It might be time for a round-up! In your opinion, what are the best
DIY projects?

Rules to keep this fair and square for everyone:

1) if you have a project of your own that supplies PCBs, layouts,
schematics, or ready modules you have to post about it. No exception
:)

2) your post needs to mention at least one project/website that freely
supplies schematics

3) we are only interested in subjective opinions, so go for it!

Of course, anything is welcome, websites, book recommendations,
youtube videos, ebooks, stories, ghost stories, etc