Smooth Proj4js integration with Leaflet
For compatibility with Leaflet 1.0-beta1 use the leaflet-proj-refactor branch.
Leaflet comes with built in support for tiles in Spherical Mercator. If you need support for tile layers in other projections, the Proj4Leaflet plugin lets you use tiles in any projection supported by Proj4js, which means support for just about any projection commonly used.
For more details and API docs, see the Proj4Leaflet site.