Nightcrawlers, wigglers, or just plain worms. Call them what you want, they are a universal bait that will work for many freshwater species. With the amount of worms I go through in a season, especially when fishing with my kids, I would be insane to buy them at the current price. With the increase in fuel prices, worms have also gone up in value around bait shops everywhere, sometimes close to $4.50 per dozen!
I've been catching them for years before big trips when I knew I'd be needing 200-300 worms, now I stock up on them every time it rains. There is nothing easier and cheaper than catching your own worms, they are always bigger and in better condition than the store bought version.
Simply wait for a rainy evening, go out about 1 hour after dark. Any lawn that has bushes surrounding it is your best bet for easy pickings. Worms will be all over the grass to, I target bushes because the worms are much easier to spot. Fill out a 1/2 liter or 1 liter container about halfway with earth. I don't use any lights to avoid spooking them. If you do need lights, cover a flashlight with yellow, orange or red plastic, the bright white light will spook them immediately.
Once you spot one (or a few), start by picking the outer ones first. A worm will usually leave 1/2 - 3/4 of it's body exposed, the "tail" will remain in the hole in case it needs to retreat. To pick it, simply pinch the tail end closest to the hole and gently pull it free. If you pinch or pull to hard, you risk ripping it in half.
Using this technique, I've managed up to 200 worms in less than 1 hour on a good rainy night, at the current prices that saved me about $75. Not too bad for an hours work, not to mention a great activity for the kids before a trip. Just be sure not to forget the OFF or Muskol.
Once you have enough worm, pop a few pin hole in the top of the containers, then store them in a fridge. Be sure to run this by your wife first, I've had early morning screeches to contend with more than once. WARNING: worms will escape if the containers aren't properly closed, last things you need is a bunch of muddy wigglers all over your food.