Nice post by Joseph Guadagno about database drivers not provided by Microsoft. He missed a few though, so here they are:
PostgreSQL needs no introductions. Probably my favourite open source DBMS. Npgsql is de facto the ADO.NET provider. Performance and stability seems alright.
PostgreSQLDirect .NET is a commercial provider, and its supposed to out-perform Npgsql while providing more features. I have never used it so don’t take my word for it.
Firebird was forked from the open source release of Borland’s InterBase. Most of you tech-oriented folk might know of it due to the name dispute with Mozilla Firefox (a.k.a. Firebird a.k.a.Phoenix) back in 2003. Its a pretty sweet product, easy to work with. I have not implemented very large projects with it, but the word is that it scales well, and has a small footprint both on disk (downloads are ~5 megs) and in memory. The only features I miss are temporary tables (coming in 2.1 which is in beta now) and materialised views.
In any case, the .NET data provider is available for download. as well as a DDEX provider.
Probably the MVP in my .NET development toolbox at the moment. SubSonic generates a full DAL based on an existing database, provides Rails-like scaffolds and provides other helpful controls and utilities. Supports SQL Server 2000 or 2005, MySQL, or Oracle (with SQLLite, SQLCE, and PostGres coming soon!).