Edit 2015-06-12. Layout updated and verified.

Here is a great sounding silicon fuzz made by Davide Gironi.
Original thread here.

EDIT 2014-10-20
Soundclip added. Clean sound running through the parallel Universe (nothing inserted in the send/return).

here is a cool circuit that reminds me of Tim Escobedo's Uglyface. It does heavy fuzz, oscillation and crazy synthy octaveish glitchy sounds.

Note that the send and return needs to be connected if you are not using other pedals through the Parallel Universe's effect loop. The easiest way would be to use a switched jack for the send and connect the switched lug to the input on the return jack. Or simply just omit the send and return and have them connected in the box. But it's when using other pedals through the fx loop you get really crazy sounds... :)

EDIT 2014-10-20
Soundclip added.

Here is a cool sounding octave fuzz by Effdub Audio. A LM386 distorts the signal and a couple of transistors in a class AB push pull configuration creates the octave up (almost identical to Tim Escobedo's Push Me Pull You, except he uses a transistor front end).

It has a strong octave up and is very simple to build. :)

Here's a very cool synthy sounding octave down / up invented by Slacker (the creator of The Echobase). Original thread here. I included the Octave up addon from the schematic aswell. I had to use shitloads of cuts and links in this layout, but at least I got it into a good size. :)

I did have problems getting this one to work/track properly. Only the low E and A strings tracked and I had to hit the strings hard. Here is what I did... Probably not the best solution lol, but for now it's good enough until I have more time to try out other things. If you want to build it 1:1 with the schematic, just omit the 47K resistor. Here's a short soundclip of the circuit/layout (with my fix):

I'm using humbuckers, neck pickup, mix at 50%, Octave up maxed and 2nd octave down selected.

Another cool effect by Robert Penfold. I could not find any info on this one so I made a layout out of pure curiosity. I was expecting a ring mod, but it's actually a cool metallic sounding octave fuzz/distortion with a fairly strong octave up and a weird stutter/tremolo mode (!) depending on how the Bias pot is set.

The only problem with this circuit is that it uses a LM565 chip which is obsolete and can be hard to find/expensive. I didn't discover that until the layout was finished (doh..) so I had to pay around 10€ for five chips from ebay.

Note that the trimmer has a very small sweetspot for the effect to work at all.

Here's an interesting Fuzz by RA Penfold from the book Music Projects (1994)
Moosapotamus has a good article on why it's a bit different from the usual fuzz.

"The Mouthmeistor" by Tiny Dazzler Electronics, a cool fuzz/wah. Have a look at my completed build here. 

Here's one insane fuzz/noise machine invented by Anchovie at the diystompboxes forum (original thread here). Have a look at my completed build here.

EDIT 2014-09-29
I added a PCB layout to the bottom of the post

Here's an early layout I forgot to post. It's the "bazz fuss" by Runoffgroove, modified with a LED for clipping and a tone control (Mark Hammer's SWTC). Super easy build and it sounds great for bass, and guitar aswell.

Soundclip:

Here's a fantastic sounding DIY fuzz from Björn Juhl of BJFE electronics.

A member of the tagboardeffects forum, John Kallas, made a better version of my layout here with the correct transistor orientation and alittle more space for the 100uF cap.