Simon Michael announced recently that his double-entry accounting tool hledger is now using Happstack. A demo is running at http://hledger.org.
Happstack Auth Module
MightyByte has released an experimental authorization module for Happstack. The git repository is located at http://github.com/mightybyte/happstack-auth/tree/master. It is based on the principles outlined in Thomas Ptacek’s excellent article on secure password schemes.