The STL file format is commonly supported by different 3D printing and modeling interfaces.
Thingiverse.com offer many models in STL format, and several popular 3D printers accept STL files to prepare models for fabrication.

MeshMagic 3D also supports reading and writing files in the 3DP, 3DS, 3MF, OBJ and PLY file formats.

• 3DS is a common file format for transferring models between 3D programs.

  Please note that all colors will be lost when saving to 3DS file format.

• 3MF file format known as 3D Manufacturing Format is a new 3D printing format designed to allow applications to share 3d models. The 3MF Consortium manages the 3MF specifications.
  The format is designed to fully describe a 3D model for the use with 3D printing applications.
  3MF is supported by 3D Builder on Windows. There are a growing number of 3MF models available for download.

• OBJ file format is commonly used to contain 3D mesh and texture data for game content.
  The meshes that have been created for non 3D printing purposes may require extra work to make them suitable for 3D printing.

  Please note that all colors and textures will be lost when saving to OBJ file format.

• PLY file format known as the Polygon File Format or the Stanford Triangle Format.
  The format is designed to store 3D data from 3D scanners.

• 3DP file format known as 3D Pad format is designed to contain 3D meshes and other properties for models mainly for the use with MeshMagic 3D.

Have a look on Thingiverse.com for some interesting 3D models.