[fro]

Check out the just released Mapbox Outdoors

https://www.mapbox.com/blog/mapbox-outdoors/

[maxerickson]

This route planner shows elevation profiles:

https://www.komoot.de/plan

I'm not sure how much it takes the elevations into account during the planning.

[Doctor_Fegg]

I've recently built http://cycle.travel/map which does indeed take elevation into account when planning a route.