A simple but flexible module that uses the great flag module for email notifications of site activity and optionally integrates with organic groups.
It uses a hierarchical notification system organised in three levels: groups,nodes,comments, where users are always notified only once for the same event, with preference to the most relevant for them (i.e. a reply to own comment is notified as such rather than "there is a comment in your group").
Flag Notify purposely do not build the flags in code (it just ask for the flags to be used), nor it creates views of elements that have been subscribed.
While this approach require little bit extra work from the users, it increases the flexibility and avoids potential problems if the og module is enabled/disabled after this module.
If necessary, I can provide an export of flags and/or views.
Default notifications for users are managed trough an option field in user profile so they can choose their default notification settings when they submit a comment, a node or join a group.
All these three "levels" are optional and if the og module is not installed the group level is just hidden.