I would be doing injustice to Wiki, if I don't share this
Here you go http://en.wikipedia.org/wiki/STL_%28file_format%29
P.S. you can use free software's like Googlesketchup or Openscad to generate STLs too.
There are many other open software's out there, but I personally recommend these.
And like I said you can also get free 3d models online, which you can then modify using the free software's; you can find 3d models at Google 3d warehouse & GrabCad.
Googlesketchup or Openscad