controller_front_init_before : this event is called before initialization itself, i.e. before any routes etc have been initialized. This could possible be used implement things like FPC and skip magento routes altogether.
controller_front_init_routers: this can be used add custom routers
controller_front_send_response_before: This event gives possibility to launch something before sending output (allow cookie setting)
controller_front_send_response_after: this can be used to set something after response is sent, like maybe logging etc