Zipping up shampoos and other liquids in your bag can keep them from leaking. You can also keep larger baggies to pack your swimsuits in, keeping everything else dry.
While you're at the beach, keep your electronics dry and sand-free in a baggie. If you're traveling with a baby, you can even pack entire outfits in Ziplocs.
On the plane, keep your medications in a baggie for easy reach. It's also a great place to put cords and chargers, and you can keep your jewelry and/or hair ties in one place.
A Ziploc will work as a sick bag in a pinch if you tend to get airsick. Also, if you buy something fragile, try filling a few baggies with air to use as packing material around it.