There are many packages for editing 3D models.
These range from high end, high cost commercial, to free to use open source, and most things in between.
A key feature is the capability to generate water tight models for export as .stl files directly compatible with slicer software, such as MakerWare.
No list can be definitive, please add your favourites below.
Inventor fusion is a solid modeller from the Autodesk stable.
It is currently a free to use technology preview. ( see warning below )
Many users who previously used 123D beta 9 migrated to Inventor Fusion when it was replaced with 123D Design which moved away from many of the features offered "to better suit the target audience".
WARNING: Inventor Fusion is a technology preview available on windows and Mac, however, the windows preview is due to expire on April 1st 2013. It is not yet clear what this means. It may be replaced by a technology preview for Inventor 360, which is a cloud based enviroment. The ease of use, compatibility of workflow are not yet known.
The Mac version has no expirey date!
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