Free STL viewer from Materialise

The Belgian firm Materialise, specializing in consultancy, services and development for rapid prototyping has released a free tool for inspection and conversion of STL files, based on their flagship Magics software. It's called Minimagics and is available for Windows.

You can use it to inspect STL files and to optimize their format. The actual tools to correct errors and optimize the geometry are only available in their (expensive) commercial tools.