1. Fuse them into a thicker plastic, and make bags and other items out of them. Tons of folks have made some really neat things out of this material. Mostly bags, but I've seen raincoats, bibs and hats too. Top of the google link to etsy's tutorial: right here.
2. Cut them into yarn. This has been going on for ages, long before 2007 when this link was posted. Still, that's a great, easy visual tutorial for ya. Then, pretty much anything you can do with yarn, you can do with plastic bag yarn. (Some folks call it plarn, but that just makes my ears want to bleed a little).
3. Use 'em to pad your fragile gifts in the mail. This especially works with produce bags and the bags that newspapers are delivered in on wet days; gently blow the bag up like a makeshift balloon, tie the end, and put it in with your item. I don't think you need a link for this one really.
4. Use 'em for pet cleanup. As in poo. It cracks me up that people buy plastic bags specifically for this purpose. I don't think you need a link for this one either. If you do, you probably shouldn't have pets anyway. :D
Okay, so it's four uses. But it's four uses I've done in the past, they're four uses that are pretty easy, and uh... you should try them too.