netfabb Studio
I think it's a very good and free tool.
http://www.netfabb.de/
[...]
Is a software tailored for additive fabrication, rapid prototyping and 3D printing.
It includes the ability of viewing, editing, repairing and analyzing stl-files or slice-based files in various formats.
Not too long ago, only high-priced software suites have been able
to perform these tasks - keeping away a large majority of the possible user base for upcoming (affordable) additive fabrication technologies.
[...]