The Lower East Kitchen has a great open source sous vide controller called the Ember. I was lucky enough to meet the LEK people at BioCurious where they did a class where we built the Ember controller and got to try some steak out of one they already built! I recommend both.
I'm not going to go over how to build the Ember kit, LEK has a great tutorial for it, (they also have a tutorial on doing exactly this but for a coffee pot). I'm going to take you though how to hack it into a crock pot. Really all we're doing here is mounting the controller, connecting 3 wires, and placing the thermometer. So even if you have a different crock pot, or something totally different, most everything here will still be of some use to you. I'm not going to go into why anything works in this; I'm aiming at a audience that has minimal electronics experience since there were a lot of people at the class that wanted a sous vide machine but had never soldered or done any electronics work before.
Step 1: Break It Open
My crockpot was held together with a wide rivet like post and a locking washer. I tried going at it a few ways but in the end I just clipped the posts off. I would have liked to reused them but they weren't designed to every come apart. I got some screws and nuts with washes to replace them.
Once its open take a look at the wires; they're should be 3 that carry power and depending on the design another set connected to the thermometer. Of the power wires, one will connect from where ever the power cable comes in to the heating element, one will go from the power cable entrance to the existing controller, and the last one will go from the controller to the other end of the heating element. You can take out the thermometer and save it from another project because the ember kit already has one its calibrated to.
Remove any mounting screws and cut all of the cables attached to the existing controller. Don't cut the wire from the power cord to the heating element, only the ones on the controller. If your wires aren't colored differently you can label them.
Step 2: Mount the Ember
Figure out how you want to mount the Ember controller. I attempted a few styles without using the existing plastic mount and going directly into the crockpot, but I was concerned around having something sticking in to far and not letting the pot fit in anymore. In the end I just used screws though the original face plate. The solid state relay is pretty big and is against the back wall of the case, so make sure you mount it in such a way that the screws won't hit it.
Drill holes in the Ember case for the screws first so you don't crask the plastic. Once I attached the box to the face plate I drilled a large hole though both pieces of plastic, this is where the thermometer and wires will go though.
Hold the Ember up to where you will mount it and figure out how long you need the wires to be to reach the ones in the crockpot. Trim off excess length and strip the ends of the wires. Also figure out where the thermometer is going to go, you might need to extend it with some of the wire you just trimmed to get it in a good place. (Try to put it in the middle, not to close to the heating element.)
Thread the wires though the hole you drilled in the Ember case and into the crockpot, now mount the ember.
Step 3: Connect It All
So the three wires coming out of the Ember are as follows: White and black are AC power, the red wire will be turned on and off depending on the temperature. So we want to connect the black wire to one of the wires from the power cable and the other to the white one. The red wire gets attached to the end of the heating element with an open wire.
You can attach the black and white wires from the ember to power and plug it in to make sure it works still
In my crockpot I was very disappointed that all the colors didn't match up. Red and white are switched from the old controller to the Ember.
Step 4: Sugru the Thermometer
Take out your Sugru and put a little but on the end of the thermometer. You want enough to keep it water proof but not so much that its going to insulate it from reading the correct temperature. This will have to dry overnight before you can use the sous vide machine. Use the rest of the Sugru on some other project, it won't last once its opened.
Step 5: Wrap It Up!
You're almost done! Because I cut the locking washers I used some nuts and bolts to tie the thing back together.
Let the Sugru set over night and then make some food!