La Nuit de l'Info - one night coding for students (one night in december). I am organizing it every year in Montpellier.
GitHub and GitHub Classroom - good stuff for education
Sage - a very easy to handle and very powerful free open-source mathematics software. Great interface for manipulating graphs!
Graphical interface for graphs in Sage
ISGCI - Graph Classes and their Inclusions
House of Graphs - Database of some interesting graphs
WolframMathWorld - small and precise definitions of some mathematical objects with examples
Sloane's database - an online encyclopedia of integer sequences
Tulip - information (graph) visualization

Research teams where I have worked