Astromech droid refers to the R-series droids. Astromechs are used for storing and executing Alliance Pilot commands in space. The following Rebel Alliance ships use Astromechs:
The Eta-2 Actis Interceptor (aka Jedi Starfighter), an Imperial Pilot quest reward for completion of Doing Your Duty, uses an Astromech.
The Alpha-3 Numbus-class V-wing and the Naboo N-1[1] also use an Astromech.
All other ships use flight computers rather than Astromechs.