How to make floating ladders

You can add invisible "barriers" and put ladders on them. See this link for more information about them.

It needs at least version 1.8 in order to work.

To get some barriers:

/give @p minecraft:barrier 64

@p stands for the nearest player (which is always you, Note: not in command blocks) and 64 stands for amount.

Also it says,

All dependent blocks can be placed on a barrier, such as trapdoors or redstone.

and

Barriers are transparent to light.

So you can make somethink look like this all the time, not sure that solid blocks are what you want, but I hope I helped you.

It can't be done in vanilla survival(without commands/external tool/switching version) in my knowledge. However, it is possible with /setblock command due to the current bug in the game which prevents block update(MC-50166).

First, pillar up some blocks with block entity (such as furnaces, dispensers, command blocks, etc.) and attach ladders on them.

Second, stand on top of the pillar and run this command repeatedly until you get to the ground: /setblock ~ ~-1 ~ air. Ignore the error message.

Then you will have truly floating ladders right in front you! Congrats!