The real revolution of 3D printing is coming from inexpensive 3D printers and from online services that will fabricate the designs you upload. The free netfabb Studio Basic software helps to create 3D designs for printing easily. It offers all the functionality needed for creating build data out of an STL file : STL display and inspection, hole closing and mesh repair, part placement and orientation on a platform, part slicing and export.
The current version is 4.0.1. A detailed documentation is available at the Wiki of the website.
The free netfabb Studio software can be upgraded to a professional version : netfabb Studio Professional. Priced at a very affordable level, netfabb Studio Professional is a full range mesh edit, repair, analysis and slicing software for a number of 3D input and output formats.
There is also a free portable app called netfabb mobile for Android or iPhone that allows you to view .STL files on your mobile device.