Brilliant piece of software. It does everything. 30 day trial.
Repair of errors in STL files
Reduction, smoothing and filtering of triangles
Cutting and splitting
Boolean operations
Offset and extrusion functions
Attach text, logos or bitmaps
Create and align base solids
Generate hollow models
import of STL, 3DS, VRML, DXF, PLY, ZPR and VFX files
export of STL, VRML, DXF, OBJ, PLY, ZPR and VFX
http://www.marcam.de/cms/viscam-mesh.84.en.html