The Minecraft Structure Planner has just had a hefty re-write and release. Here's what you can get in the new version:
GUI Redesign: The GUI was becoming cluttered with all the different model tabs, so now it will only open and display tabs for models that you request from the 'Structures' menu. This also has the side benefit of allowing you to design multiples of any given model at the same time.
3D Performance: The rendering engine has been re-written for better performance, and has also had a major Java 3D memory leak corrected.
More Textures: By request the texture support has been increased to cover Nether building materials, as well as plants, chests, furnaces etc.
Circle Planner: Another request, the Structure Planner now has a simple Circle Planner tab. Specify the radius, and it'll give you the single layer plan for that circle. You can see an example of the new window here.
Torus Planner: Another requested model, you can now specify and plan out giant donuts to your heart's content. Screenshot here.
Plan Scaling: Yet another request, there is now a scale slider at the bottom of the plan window (you can see it in the 'Circle Planner' screenshot.) This allows you to change the on-screen scale of your plans from 0.25x to 2x normal scale, allowing you to more easily see your layer-by-layer design layout on both smaller and larger designs.
Save/Load/Print Hooks**: Save and Load now save your structures to an XML based file format.
Java requirements reduced to 1.5: You should now only need JRE 1.5 or higher to run the Minecraft Structure Planner, rather than the previous requirement for JRE 1.6.