I just finished building a Splitter-Blend from Run Off Groove, You can find the schematic at runoffgroove.com. It basically splits your signal and runs each side chain through 2 parallel effects loops then mixes them back together for the output. By putting different effects in each loop you can blend them to achieve lots of unique tones.
I also added a few simple mods that make it more useful. I used normally closed jacks connecting the input and output for each loop, that way you can put your effects on one channel and the other channel automatically sends through a straight signal. The connection is broken when you plug into any jack. Blending in the straight signal with the effected signal works especially well on bass, because you can add a little uneffected signal that lets you retain the bottom even on effects made for guitar. Adding some straight signal can also add a little clarity to a fuzz tone or you can make a phaser like effect by slowly rocking a wah pedal in one of the loops. I also put the phase switch on a DPST foot switch so the phase can be inverted when playing. The blend pot knob is from a junked toy truck and allows you to use your foot to adjust it (henceforth to be known as the "EarCo Rock-N-Roller Wheel TM" using EarCo's unique "Speedwheel TM" technology). The Rock-N-Roller Wheel TM makes it easy to rock back and forth between effects setups.