Atomic vertical-axis wind turbine.

I made a few refinements. This new shape is designed for vertical stacking up to whatever height is desired, with interlocking pieces to help maintain structural integrity and reduce vibrations. This is just a tweak to the previous design - the new arm isn't so strong, but it does eliminate that really nasty overhang. I have tested it out now (Thanks to some suitably British weather) and can verify that it does spin, and at a pretty decent speed too! Though without much torque.

This design works in two- or three-blade configurations. Unintuitive as it may seem, three blades isn't actually more powerful than two, except at very low wind speeds. Three blade have a different advantage: They run a smoother. Less noise, less vibration. A nice property of this design is that you can print a two-blade turbine out for testing, and if it shakes two much re-pupose the sections for a three-blade.

There's a catch to this design: It doesn't start easily. It's fine under no-load, but if you have any resistance to movement at all the it isn't going to start turning. You can hang it, or put it on a smooth rod, or build a pole with bearings to put it on.

You can print a turbine about 50% wider than the long diagonal of your build platform. That's as big as is practical for a printed turbine.

Note that you're never going to get high power output from a wind turbine unless it's really huge - but 3D printed models are fine for decorating the garden with and testing out designs. Sorry, but physics is against you: The power from a wind turbine increases with the square of the size.

Perhaps someone can find a way to make it useful - you can't really generate power off of it, but it could be used to drive a small music device that rings bells in sequence, or act as a bird scarer.

Now with demonstration video:
Mk7: https://www.youtube.com/watch?v=l2KNNG8fuXA
Mk10: https://www.youtube.com/watch?v=XcxPIb6TwHY