3D Printing is perfect for augmenting an existing product to improve its function and prolong its life. A perfect example is the 3D printed Sprout by Egant on Shapeways, that transforms the humble milk jug into an elegant watering can with some clever 3D printing magic.
The innovative design has two flow types and uses a captured ball valve so that the center can be used as a funnel yet the ball stops the water from pouring of the funnel aperture when you want to have a steady controlled flow. This is really smart use of the potential of interlocking and/or captured parts that is possible with Nylon 3D printing whilst upcycling an existing product to prolong its lifecycle.
Check out the video which sets a new benchmark on Shapeways for an instructional video.
That is a truly awesome design. I love the fact that it takes advantage or the 3d printing bypassing manufacturing limitations with the rolling ball valve. Very elegant solution for reusing the jugs. Of course it's only useful for those who buy their milk/water in those jugs, but I still love it!!