This pugin allows you to turn any block into a slab.
It is based on SimplySarcs "Turn Any Block Into Half-Slabs" concept.
Also check out AdvancedSlabs if you are on 1.9!
- /anyslab create [solid] - starts the creation of a new slab (Permission: anyslab.create)
Create a slab
To create a slab simply execute the command above. Then you can right-click the block you want to turn into a slab with the block representing the material of the slab.