I'm focusing on a wide open source project which could use numerous DBMS' as after sales. To date we supports SQL Server 2005 and SQL Server 2008 but we'd like to aid some additional databases including some file based DBs.

Note: The file based databases must focus on a 64-bit platform, so MS Access isn't a choice as there's no 64-bit provider AFAIK.

Which DBMS should a database centric free project support inside your opinion?

SQLite, certainly.

Provide ODBC support, then your finish-user can decide on nearly any database going.

Firebird, in both its embedded variant or like a (small footprint) server.

MySQL, it is a broadly used free/free RDBMS.

Not really a file based DBMS, however, you did not say it needed to be, that you desired a couple of of individuals too.

Support these: sqlite,mysql,postgresql

You do not mention what language you are developing in. Some, like Java, have standard database APIs, so supporting multiple SQL servers is simple, provided you should use the most popular denominator from the SQL language inside your application.

To be sure that firebird could be much better than SQL Lite and M$SQL Express , as well as is preferable to mysql , scales better with the amount of Processor chips and it is elderly (triggers , and transactions are implemented for a lot of many many years ago)