Surely just do what ever you'd try to do with another language but just use Haskell instead.

Depends on how complex you want to do things. You can just use yesod [0] to create a web app and compare it to an existing one you'd written.

An interesting use of Haskell is with concurrency [1], so anything along those lines should give you something pretty awesome.

[0]: http://www.yesodweb.com/

[1]: http://chimera.labs.oreilly.com/books/1230000000929/index.ht...