InvisibleWalls fixes invisible ladders & iron doors for 1.8 clients by replacing them with barrier blocks which are only visible for 1.8 clients.
Without the plugin (1.8 client)
with the plugin
(replaced with barrier blocks)
This plugin depends on AlternateBlocks
#Replace iron doors
#Check for ladders based on attached block on their back faces
#Check for ladders based on the relative position to players