Since I remember there have been free access electronic circuits, I remember electronic magazines including several projects each issue. But lately this trend has gone further.
Internet has given normal people the capacity to put things so that others can check them, so when applied to electronics we find a clear improvement: we are not limited to the things we can put in a magazine, neither are we limited in the level of knowledge needed to use the electronics, as usually are the magazines that are intended for a certain audience.
So new ways of distribution of knowledge are opened as a lot of circuits more or less completed, more or less complicated, are published with freedom in mind, with the freedom to use them for whatever we want (indeed, several of this projects allow even commercial applications).
Thus it's born the category known as Open Hardware projects. It's a step beyond of the Open Software concept. Here communities are born around the use, modification, improvement and redesign of advanced circuitry. There are several sorts of circuits, ranging from general purpose to ultra specialized, from a big majority of exposed circuits to a full fledged modern cellphone.
I'm a programmer, so I admit I only made baby steps into electronics, that's why I can't do anything but admire this marvelous things we are freely offered. We are given the freedom of buying them, of buying components and building them. We also have the freedom of developing from them and, if we want, we can release our modifications to the community, thus making bigger the list of Open Hardware devices and their capabilities.
A little advice: the page where I dicovered this is ordered by alphabetical order, so you'll see a lot of Arduino devices first. Don't worry, there are a lot more things below.