While screwing around with various XOR gate designs, I came along with this one. I assumed this would be the best place to post it, instead of just creating another thread. I do not know if this design has been found, as it is incredibly simple. Also, I put the images in spoilers, because they're in a large resolution. I hope it's moderately helpful as it's not very large and as far as I can tell; it's instant. However, the downside is that the outputs are not connected to one another to form a single output line. Once again, I apologize if this design has already been posted.
Edit: In my infinite stupidity, I forgot to place a block necessary for the gate to function. I have now fixed and updated it.
caramba: The fact that it's a sponge based one is what discouraged me from posting it in the first place. Can't really get sponges unless you're haxxing. Granted, that's what most people here do anyway.
I'll prolly add it after I'm done with the other ones, but it's gonna be low priority.
Anyway, changed clock #2 into a more efficient one.
It uses two less blocks if you consider that redstone has to have a block underneath it.
It is also easier to connect to something e.g. a piston can be placed right next to the red block.
In XOR #4 the output is the black block diagonal to the red one.
Updated XOR #4, thanks.
As for the most "basic" clock - doesn't really matter which one to use.. The current one uses less redstone functions (doesn't feature repeater powering a block which sends that power over to the dust next to it), therefore it's easier to understand. I highly doubt anyone's gonna care about saving two pieces of dust.