Nice post by Joseph Guadagno about database drivers not provided by Microsoft. He missed a few though, so here they are:
PostgreSQL
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
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.
SubSonic
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!).
ActiveRecord
An implementation of the ActiveRecord pattern by the Castle Project, built on top of NHibernate.