Which databases are recommended to be used as embedded databases to store data within an application? The embedded database may or may not synchronize back to a larger system database.
A list can be found on Wikipedia. I think the overwhelming leader in embedded sql databases is sqlite. Found everywhere!
SQLite is probably the most commonly used one. It is light weight and has an open license. SQLite is being used by many embedded systems including Apple's iPod/iPhone/iTouch line of mobile devices, Google's Android phones, and Airbus aircraft.
Given this I would probably recommend its use, but I will say that I myself have not used it for an embedded system.