[samuraiseoul]

A early facebook clone can work well. If you're learning a new frontend tech, reuse the backend from one you did before, or if it's new backend, reuse the frontend from a different one.

I say early facebook cause I feel that doing a login, and feed page, and allowing status updates, friends search and confirmation, and a profile page with photo uploads to be a pretty good intro stack.

That said for a new language I ALWAYS look for a modern koans exercise to do. Some langs have them, some don't.