Thanks Peter. Unfortunately, it syncs with Toodledo which means I can't use it.
Perhaps I didn't explain myself very well. When I say offline, I mean no information can leave our network. Group collaboration would have to be done via something like a network share or a locally installed syncing service.
'Chandler is an open source note-to-self organizer designed for personal use and small group collaboration. Chandler comprises a desktop application, sharing service and web application.' (from Chandler FAQ)