General umbrella Facebook group. Ask questions here if you can't find answers at region-specific groups.
For easiest searching and orientation. All the spots on it are also under 'Places' on this site.
Basic info to get started, with brief profiles of camps and lists of which camps need which language skills.
Good overview of what to expect as a volunteer -- mental prep, rather than logistical. Compiled by Campfire Innovation.
Maintained by Bienvenidos Refugiados en Grecia.
Compiled by Sumita Shah. Instructions for (highly recommended) procedure of getting official clearance to work with children.
Right here on this very site.