External Chargers

HTC One X. Nexus 4. iPhone. Nokia Lumia. Optimus G.

All these devices share a common thread: Non removable batteries.

For many, these devices can manage through a good chunk of the day, but what about those days where you may be away from a charging outlet? Airplane rides? Long car trips where everyone is rationing the single outlet? What then?

Since removable batteries seem to have become a dying breed, what external battery packs would you suggest?

Please provide links if possible.