Intro: Restore a Rusted Shut Tool
Step 1: Supplies
apple cider vinegar (white vinegar may work as well, though I haven't tried it yet)
salt (not sure if this is entirely necessary - but I know it works well for cleaning pennies in concert with the vinegar)
plastic dish big enough to submerge the rusted parts of the tool
Step 2: Submerge the Tool in Vinegar
place the rusted part of the tool in the dish
pour in enough vinegar to submerge the rusted parts
Step 3: Add Salt
apply salt liberally over the whole area
Step 4: Check on It Tomorrow
Leave the tool in the mixture for 24 hours
Step 5: Brush
After a day has passed, look at the tool. You should see a lot of rust flakes and debris in the solution.
Use the old toothbrush and scrub away the remaining deposits.
Step 6: Wiggle
If that doesn't work, maybe leave it another 24 hours. But this treatment should be enough to get it back into working order.
Step 7: Blot, Dry, Oil
Give it few drops of oil and work it to help keep it from getting rusty again. Not sure what kind of oil is appropriate here, I had this 3-in-1 oil handy so I used it. Some commenters are swearing by WD40. Some people are saying that WD40 is green, however, it looks to be made of mostly petroleum products: http://www.wired.com/science/discoveries/magazine/17-05/st_whatsinside
And, since everybody keeps mentioning it: another way achieve a similar result is with an electrolytic process. If you have the facility to do that, more power to ya.