Do you have an old PC fan that runs really well, except for the fact that it's missing a blade? Well do not worry, you can fix it with a soldering iron and a little patience.

What you need:
Soldering iron
Electrical tape
PC Fan, make sure you have the missing blade(s)

Notes and warnings: Make sure you have plenty of ventilation. I'm no toxicologist, but I highly doubt you're supposed to breathe any fumes coming off of molten plastic. I used a (working) PC fan, and it seemed to work fine.

This may damage your soldering iron, so I'd suggest using an old one you don't mind ruining. Some irons (like mine) don't seem to care though.

I take no responsibility for damage of property or physical injury / death, although I'm pretty sure you can't die from something like this.

Step 1: Prepare the Fan for Operation

First, you have to put a small piece of electrical tape on the blade, then carefully tape the blade back onto the shaft. Make sure you line up the cracks, or else the fan will be unbalanced. How exact you have to be depends on the fan's speed. Slow fans (1500 RPM >) vibrate less when unbalanced, whereas faster ones (1500 +) vibrate like crazy. But whatever the speed, you should be going for exact anyway.

Next, you have to melt together a small corner of the blade. While supporting the blade (VERY CAREFULLY) with your finger, use the soldering iron and join a small corner in the crack. After the joint cools, you are ready to proceed.

Step 2: Melt the Crack Back Together

Starting with the side that is not taped, use your soldering iron and slowly run it along the crack. The plastic should melt and fill the crack. Then remove the tape and melt the other side. Carefully inspect the repair and melt any spots you missed.

After the plastic cools, gently press down on the blade. If it snaps off, then start over, moving the iron slower. The more plastic that melts, the better. But make sure you don't go too slow, or else you'll end up warping the blade. If you are satisfied with your work, then plug the fan into a power source to test it. Run it for about 5-10 minutes to make sure the blade won't fly off. It'd probably be best to be behind something while you do this.

Step 3: Rinse and Repeat

Repeat these steps for any more missing blades. And voila! You have just revived a perfectly good fan. I did this to a 3000 RPM fan, and now I still get to use it. Maybe next time I won't stick something heavy in it while it's running.