* SQLite - it's everywhere thanks to mobile phones.
* Oracle.
* Microsoft's thing has to have a big following.
* Mysql.
* DB2 is not as big as Oracle, but IBM isn't some lightweight, either.
MongoDB gets talked about a lot, and probably used among people trying new stuff out, but I think a lot of mostly silent people wouldn't touch something like that for their important data with a 10 foot pole.