They are called [drop tanks][1]. When a drop tank is jettisoned, very likely it is **not** retrieved and reused. This is also the case with (sadly, discontinued) Space Shuttle [external tank][2]. Those tanks were [not reused either][3]. I am not talking about the [rocket boosters][4].

**What happens to jettisoned fuel tanks?**

It depends on where they land. The tanks jettisoned in Vietnam have been used to make boats (more [pictures here][6]).

It is worth noting that these external [fuel tanks have disadvantage too][7]:

> A general rule is that only about half the capacity of a streamlined
> drop tank actually goes towards increasing the aircraft's overall
> range, the rest going to overcome the added drag and weight of the
> tank itself.


  [1]: http://en.wikipedia.org/wiki/Drop_tank
  [2]: http://en.wikipedia.org/wiki/Space_Shuttle_external_tank
  [3]: http://space.stackexchange.com/q/3616/1344
  [4]: http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster
  [5]: http://www.viralnova.com/vietnam-fuel-tanks/
  [6]: http://www.dailymail.co.uk/news/article-2612962/Ingenious-farmers-build-vital-river-boats-fuel-tanks-jettisoned-U-S-planes-Vietnam-War.html
  [7]: http://en.wikipedia.org/wiki/Drop_tank#Overview