Water balloons are sort of a pain to tie off after getting filled, I guess. I mean, I've never had that much trouble with them, but if a complicated gadget exists to make the process easier I assume somebody's been struggling.
The Tie Knot Water Balloon Filler fits onto the end of your hose and lets you fill up and tie off a balloon in under 30 seconds. It lets you clamp down the balloon's neck so you can easily tie it up without the pressure of water trying to get out. Makes sense! It's also only $10, which isn't too bad, but is also $10 more than just tying the damned things off yourself. Your call!