Update usefullinks.rst

Author: villekf

docs/source/usefullinks.rst

@@ -142,6 +142,9 @@ Data analysis software
 * (X)MedCon: https://xmedcon.sourceforge.io/
  * Medical image conversion tool
 
+* 3D Slicer: https://www.slicer.org/
+ * 3D Slicer is a free, open source software for visualization, processing, segmentation, registration, and analysis of medical, biomedical, and other 3D images and meshes; and planning and navigating image-guided procedures.
+ 
 Programming and languages
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