Lost Eidolons has CP exchanges with the following LARPs.
The Calling http://www.thecallinglarp.com
Endgame http://www.endgamelarp.com/
The Isles http://www.theisles.org/
Madrigal http://www.larp.com/madrigal/
Seven Virtues http://www.carusoking.com/sevenvirtues/Seven_Virtues/
Pirate Island http://pirates.larp.com/
Aralis 2 http://www.aralis.net/
Terres Rising http://www.terresrising.com/ Non-Accelerant game that allows latex weapons
Requiem http://www.requiemlarp.com/pub/home/home.php
End of Seasons: Veneration http://lione5.endofseasons.com/
NERO Boston: http://www.neroboston.com/
This list will be updated as CP exchanges are formally established.
For the uninitiated, a CP exchange is when you can earn Character Points towards your player character at a given LARP by helping to run a different LARP (generally by attending the second LARP as a non-player character, e.g., a monster).