In addition you will find the Caisse Auxiliaire, the Caisse Soins de Santé de la SNCB and the CAMI - Caisse Auxiliaire d'Assurance Maladie Invalidité (public organisation the gives benefits in case of disability without offering additional mutual insurance coverage). Some insurance companies have equally a department named "mutual".
You will find more information on the website of the Institut National d'Assurance Maladie – Invalidité (INAMI): http://www.inami.fgov.be
Mutual benefit society pay also part of the price of some medecines when they are bought with a prescription with the sticker "mutual" (mutuelle).
There are also additional healthcare insurances, non mandatory, that can refund the amount not already paid back by the mutual benefit societies.