To the list, A few comments regarding the frozen stem. Like Dale I see these several times a year and 'am asked to remove them, here's my take. Rule #1--- never over stress the frame or fork to remove a frozen stem, or seat post!!!!! Even if its a gold plated one pantographed by the old old man himself.... The frame comes first in my book. Use all of the penetrants already mentioned, give them time to work, apply moderate (not brute) force, and if it doesn't work quit while you're ahead and call a professional!
When one of these come into my shop I'll soak it over night, then try moving it , if it doesn't go I'll repeat the process several times, and may use a little heat and cold also. If its still stuck I'll cut it like Dale says and then make several internal saw cuts, carefuly! and then remove it. Not my favorite thing to do but if the frame is saved there is a certain satisfaction to the process.
There's a neat trick that sometimes works on seat posts (don't try this on stems o.k.) Remove the binder bolt , spray it w/ (your choice of penetrant gleaned from list postings) and go ride the bike for a few days, spraying between rides. Remember to take the binder bolt and wrench w/ you in case it works. I've seen this work many times, the pressure, and rocking motion do the work for you.
I've had many frames and forks sent to me by ridersand shop after they went too far... I've had to replace steerers, seat tubes, seat stays, lugs (then paint them), and do alingments on frames that were brutalized and violated,(don't know they just sounded good together), the cost to repair far exceeded the cost of the part being saved.