Key to getting the best, healthiest, and most yeast cells is oxygenation. Short of having an oxygen tank bubbling through the mixture (which I could in fact do, but dragging my welding cart in to the kitchen might result in objections, and it's too cold for the yeast out in the garage) frequent stirring is the next best method. So every hour (or whenever I happened to think of it, which was in practice a lot less frequent than hourly) I'd give the container a thorough swirl; the yeast clearly loved it, but it was a pain to have to do this repeatedly. "I wish I had a stir plate" I said. I've said this before. Yesterday I decided to get ready for the next time.
A magnetic stir plate is really pretty straightforward. It's a motor, with a magnet on top. Now you can get fancy and add speed control, or a heater, or a nice case to put it in... but the basic function is simple, and I figured the parts were already lying around the garage.
I had several old computer fans in a box, so I grabbed the biggest one. They're already speed-controlled to about the right speed for this. Hm, what has a big magnet? Why, an old hard drive of course! They have a couple of super-powerful magnets, and I have a big stack of dead hard drives from which I could scavenge. I also keep a box of assorted power supplies, so I easily found a 12VDC wall-wart to wire up to this. A few screws from the hardware store (but only because I didn't have anything long enough lying around), some foamcore from the craft closet, and a layer of Rapid Ingenuity Tape (Thanks Greg!) and voila - one ugly but highly-functional and super-cheap stir plate! Ta da. A thingy.