[skew]

You haven't got many fun answers yet? Here are a few nice programs

http://incredible.pm/

http://fluffynukeit.com/software/fnistash/

http://hledger.org/

If you're looking for not-necessarily-polished personal toy projects, probably just about everything in Hackage's "Game" category fits:

http://hackage.haskell.org/packages/#cat:Game

Monadius has an interesting look. Sadly a touch of bitrot since 2013 - you'd need to `cabal get monadius` and edit

    --- Monadius-0.99/src/Monadius.hs	2013-05-28 
    +++ Monadius-fix/src/Monadius.hs	2017-01-25
    @@ -18,5 +18,5 @@
     import Data.List
     import Data.Maybe
    -import Graphics.UI.GLUT hiding (position)
    +import Graphics.UI.GLUT hiding (position, DebugMessage)