I love zip lock bags. My house has boxes of them spread all over the place. I save all the ones that things come packaged in - wee little ones and great big ones. They're great for keeping cotton wool balls and buds dry in the toiletries bag when you're travelling; storing buttons or bits you want to take shopping to match against things. I have two favourite uses for them though, and generally use the biggest ones I can buy.
Use #1 in the sewing room - I store all the bits and pieces of a work in progress in a zip lock bag. If I buy fabric for a particular project I store it with the pattern in a bag so I don't accidently use the fabric for something else. Once I start cutting up zip locks come into their own. So handy.
Use #2 (so different!) in the kitchen - zip lock bags are brilliant for marinading and coating food.
To marinade: make the marinade in the bag (no washing up!), squish it around, add the meat (or whatever), squish around, fold, put in the fridge. Every so often, squish it around again. If you're wary of the seal of the bag - put it on a tray.
To coat: add all the ingredients to your bag, add meat, squish around. I usually blow a little air into the bag, seal, and then toss it about. A lovely coating for chicken - garlic powder, onion powder, paprika, and equal quantities of breadcrumbs (or cornmeal or crushed cornflakes) and flour.