A regular TileLayer with black and white makeover.
Based on the grayscale TileLayer by Ilya Zverev
Just use L.tileLayer.blackAndWhite(url, options) instead of L.tileLayer(url, options).
Ilya recommends using fadeAnimation: false option when creating the map object. Otherwise you will see a lot of flickering.
http://aoloe.github.com/leaflet-blackAndWhite/
The original plugin was written by Ilya Zverev and published under WTFPL license.