When looking for an open source database give a try to Ingres.
It's the very first RDBMS product, very mature and reliable.
It has a fine transaction engine,runs on many platforms, has many connectivity options such as native .NET data provider , DBI etc and has many features such as writing ESQL/C programs, ABF/4GL and Openroad a tool for rapid development which can also be used to bring rich applications on the web.
It's left behind marketing wise because as a company they had structuring problems but now they released it on open source.
I've been using II 2.6 on Unix for 5+ years now and although the version I use is old and lacks features I still find it a solid product.But the new Ingres 2006 is a different proposition all together.
in reply to [OT] Why I don't use Mysql for new projects