Links:

Institutes:

Scaliger Institute
AHRC Centre for Early Modern Lives and Letters
Society for Renaissance Studies
Huygens Institute
Erasmus Centre of Early Modern Studies
Amsterdam Centre for the Study of the Golden Age
Centro de Estudios Humanísticos
Renaissance Society of America

Neo-Latin Associations:

International Association for Neo-Latin Studies                
Cambridge Neo-Latin Society                                            
Seminarium Philologiae Humanisticae                              
American Association for Neo-Latin Studies

Online sources for early modern letters

See the bibliography on this site.