Postgres is a full featured database that is very actively supported as Opensource. The license is such that it is completely free under all circumstances, unless you choose to buy a supported version. There are a number of books available for PostGres and there is a is a special postgres library for .net. Runs in all environments and scales at least equal to mssql. Does not require special server software.

We are going to be using MSSQL Express for our converted VFP program, but it is NOT my choice because of its limitations and the eventual expense to moving to big daddy.