Maybe you have just scanned an object and want to print it on a 3D printer. The model has to be water-tight and most 3D programs don’t have “the right tools” to heal 3D meshes easily. In this tutorial from Shapeways you can learn how to use the open source 3D software Meshlab and the German 3D program netfabb to fix a mesh before 3D printing it. Both software packages work perfectly under Linux OS.
MeshMixer is a 3D software that kan make one smile and laugh and have lots of fun while mixing any kind of mesh and 3D shape. It does stuff that most 3D programs can’t do and some features resemble those of very expensive commercial programs. MeshMixer is free and even though it is a Windows program it runs well under Wine and Linux.