Here's a cool  auto wah from Robert Penfold that can sound really funky and bubbly. 
In the soundclip the resonance and depth are maxed.

I have adapted it for true bypass and added a volume pot since the output was alittle bit above unity. The controls are a bit finicky, especially the frequency controls that goes from low (so low that it kills the signal) to above the hearing range. I think a log pot work better here. I used a 2M resistor insted of the 1.8M and a BC108B transistor with good results. The 10K trimpot sets the input for the envelope follower. Adjust it until it sounds good and you get a "natural" sounding release.

Phozer version 1, with a few small modifications. It's gives a nice modulated wah type sound.

Note that this effect needs the rate indicator LED to complete the circuit.

The Phozer 2.0 is a great sounding psuedo phaser by Runoffgroove.

Here is another filter commonly found in hifi amplifiers that I adapted it for use with 9v single supply and guitar signal. In the middle of the tone pot it is almost completely flat and to one side it boost below the center frequency and to the other side it cuts. It also works great as a treble booster or a bass booster (depending on how you set the level), both as a stand alone unit or after a low output fuzz. :)

I recommend putting the level/gain pot outside the box insted of a trimmer in this one.

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

Here's a nice active Baxandall qualizer that I designed. Originally I did a layout for a schematic called "Bax in a box" (google it and you'll find it) that worked ok, but it was just "semi-active". The dual op-amp was just acting as a buffer and a make-up gainstage, so this is my take on an improved truly active version.

 You can put after your favourite fuzz or whatever, or just in a stand alone box.

Here is a very cool optical filter/tremolo by Dead Astronaut. Built and verified today - works perfectly. :)

Here is a simple but very cool sounding auto wah by Tim Escobedo with a few mods by Dead Astronaut. Unfortunatly I didn't get the down mode to work as per Dead Astronaut's mod, so I omitted it (listening to sound-examples I didn't like it much anyway) but I kept his other mods (a few value changes and a better release/intensity pot).

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