Oh man, those things are craaaaaaaaazy useful. Here's a bunch of ideas to get you going.
Make a bird feeder. Cut a hole in two or three sides of the jug. The holes should be 2 or 4 inches in diameter, depending on the type of birds you want to attract. For perches, make smaller holes below the feeding holes. Push wooden dowel rods through the holes so they poke through the opposite side. Fill the feeder with seed to weigh it down, then top the sand with bird feed and hang from a nearby tree. You may want to poke a few small holes in the bottom for rainwater drainage.
Cut the bottoms off and use them as plant saucers.
Make luminaries by cutting off the top of the jug, filling the bottom with 2 or 3 inches of sand, then placing a votive candle in the sand. Line a walkway or garden path with the lights.
Organize the refrigerator. Cut the tops off several plastic jugs and use the bases to conveniently store grapes, kiwis, pear tomatoes, cheese, lunch meats and other small items in the fridge. Make shorter containers for the storage drawers and trays. Works for other drawers as well!
Cut the bottom from a plastic jug just under the handle, and use the top part as a funnel for liquids, powdered dry goods, flour and more. Leave the cap on (tightly) and you've got a scoop!
Cut a large hole (big enough to put your hand through) in the top corner opposite the handle, and use them to store things in, such as craft odds and ends, votive candles, or anything else smallish.
They're also great for storage as they are. Wash them out (hot water and a little soap, cap, shake like hell, and rinse really really well), leave them upside-down to dry, and then use to store bulk dry goods. Better than plastic bags, and easy to pour. Also works as a pitcher of you want to make a gallon of juice/tea/etc to keep on hand.
Loose change jar! Just cut a slit in the top corner opposite the handle and feed your loose change in. When it's full, roll the change and put it aside! It adds up really fast, and makes a great "fun fund" to use towards vacations or goodies for yourself.
Don't forget to decorate them and jazz them up, so they aren't just plain ol' plastic jugs anymore! Paint, decoupage, markers, ribbons, whatever strikes your fancy. And post pictures of what you decide to make!