Haskell

Writing an interpreter is the classic Haskell project, and the list has three. Write Yourself a Scheme in 48 Hours, a Wikibook by Jonathan Tang, builds a Scheme interpreter with Parsec and teaches monads, error handling and IO through a real program. Write You A Scheme, Version 2 by Adam Wespiser updates the same idea with modern Haskell practices and tooling. Stephen Diehl's Write You a Haskell goes much further, building a compiler for a small functional language with type inference, though it was never finished.

For something practical, Roll Your Own IRC Bot on the Haskell wiki builds a bot that connects to an IRC server and responds to commands, and is short enough to finish in a sitting. Making a Website with Haskell builds a small web app. Making Movie Monad by David Lettier builds a desktop video player with GTK and GStreamer.

A sensible order is the IRC bot, then Write Yourself a Scheme, then the website or the video player, and finally Write You a Haskell once you are comfortable with type classes and monad transformers.