I just bought a PennPlax Automatic Fish Feeder for about $10-US here in my LFS.
Feeds twice a day
Large capacity drum
Uses 1-AA battery
Clamp on or free standing
The autofeeder is a bit smaller than the others I've seen so far. You only need to use 1 AA battery to run it. It functions really simple. It rotates every hour, just like the hand of the clock. When it points to the 6hr mark (downwards), it collects food, then when it reaches the 10hr mark (leftwards), it dumps all the food it collected into your tank. Simple! To adjust when to dump the food, you only need to turn the drum and set it where you want it to dump.
It worked wonders for me. To test it, you just need to pull off the drum that holds the food and rotate it on your own. This way, you know for sure how much food it will dispense every 12 hours. To change how much it dispenses, you just need to turn the knob clockwise and counterclockwise. It's as simple as that.
After a while of inspection, a few problems stand out. Firstly, you don't exactly know when it will dump the food, just an idea on which hour it will do it. There's no way to know unless you wait. Another problem that appears is that the food supply is not that constant. If you fill the drum all the way up, it will dispense a really huge amount of food. When it gets down to about 1/3 its capacity, it will dispense a smaller amount of food. So you really have to keep an eye on it.
This product is very good for anyone who can't feed their fish at the right time or has to go out of town and doesn't have anyone to feed their fish. But for me, it's just another toy for me and my fish.