For most caring divers this means an "octopus" spare second stage on an appropriately longer hose. The best time to buy an octopus is when you buy the main regulator.
Second stages sold as octopus models are generally tuned to resist free-flow, as they spend most of their time in the water with no mouth attached to provide the necessary back-pressure.
If you have chosen a coldwater regulator, logically you need a coldwater octopus. Avoid the temptation just to use an old second stage without bringing it fully up to spec and adapting it for its new role.
To be usable in an emergency an octopus needs to have a hose at least 1m long to allow comfortable face-to-face or side-by-side ascents. The extra-long hoses favoured by some technical divers foresee the need to pass the octopus to someone while swimming through restricted passages.
If you opt for a minimum-hose configuration with a combined BC inflator/octopus such as the Apeks Octo+, the SeaQuest Air Source 2 or Scubapro Air 2, remember that the emergency scenario involves you handing over your main regulator (on its standard-length hose) to your buddy while you breath from the AAS.
Not listed here are the increasingly popular pony-bottle arrangements - see the cylinder listing - used with a standard regulator.