If you ever need to clean a peanut butter jar here is a quick way.
Step 1: Egg Shells
Toss some egg shells into the jar. Two or three should do fine.
Step 2: Water
Add a little warm water from the tap. Fill the jar about 1/3 full and put the top back on.
Step 3: Shake
Shake the jar for about a minuet or so. The egg shells will break up and scrub the sides of the jar. Try to get the water to slosh back and forth, not swirl.
Step 4: Rinse and Repeat
Pour the water and shells out into a strainer of some sort. Notice how the shells broke into smaller pieces. At this point you might want to repeat step 3.
Step 5: Finished:
Now you have a nice clean jar. There might still be some residue left but it will wipe out pretty easy. At lest a lot better then before you started.