With the major overhaul of Hammer in Source 2 it is also quite simple to create custom helpers/visualization for entities.
For example, here we have a helper showing the opened position for the door. This is something I know a lot of mappers have struggled with in the past, so should cut out a lot of iteration.
Our intention is to eventually allow coders to add their own Helpers and Tools to Hammer - but that's a while away.