Was given a really groovy microwave oven; groovy except it really
stinks from food that was burned in it previously. Doesn't seem to
effect food that we cook in it, but how can I get rid of that smell? 
I've tried baking soda in every form; no success.

I have removed many smells from a microwae I recieved by doing the following. 
1. Remove all internal components from the microwave ( revolving tray ext)
2. Clean using "Purell Hand Sanitizer" I know it sounds wierd, but it worked
3. after wiping every area of the micrwoave with Purel dray and let
the microwave remain open for about 2 hours.
4. Clean the internal compoents you removed with either Purell, or
just soak in regular dish water soap.
5. Re-assembly the microwave and you should notice the smells gone.
best of luck - ted

Whatever you use to clean it, pay special attention to the seal around
the door. The rubber harbours odours readily.

thanks, you two.

I did it. It worked!