Tinkercad is a FREE, easy to use, web-based, 3D modeling application geared toward kids in K-12, but anyone can use it to make handy and fun objects that can be printed using a 3D printer.
This Getting Started Project (PDF) walks you through the steps to make a boat, complete with fun cutout shapes for windows. Print it out, or read it on-screen, and start building!
Google's 3D Warehouse has 10 puzzle models that you can download. This page shows you how to find and download the models, and also shows how to solve them.
These puzzles are made in the book ModelMetricks Intermediate Series Book 3: Puzzles
Google's 3D Warehouse has 52 mosaic models that you can download. This page shows you how to find and download the models, and shows several ways you can color them (including printing them out).
These mosaics are made in two books:
GeomeTricks Periodic Patterns Book 2: Mosaics with Regular Polygons