I am working on an open source project which can use a number of DBMS' as backend. So far we supports SQL Server 2005 and SQL Server 2008 but we would also like to support some additional databases including some file based DBs.
Note: The file based databases must work on a 64-bit platform, so MS Access is not an option as there is no 64-bit provider AFAIK.
Which DBMS should a database centric open source project support in your opinion?
Firebird, either in its embedded variant or as a (small footprint) server.
MySQL, it's a widely used free/open source RDBMS.
Not a file based DBMS, but you didn't say it had to be, only that you wanted a few of those too.
Provide ODBC support, then the end-user can choose from just about any database going.
Support these: sqlite,mysql,postgresql
You don't mention what language you're developing in. Some, like Java, have standard database APIs, so supporting multiple SQL servers is easy, provided you can use the common denominator of the SQL language in your application.
I agree that firebird can be better than SQL Lite and M$SQL Express , or even is better than mysql , scales better with the number of CPUS and is more mature (triggers , and transactions are implemented for many many light years ago)