.NET Data Access Objects
.NET Data Access Objects is a loosely coupled, extensible database access library written in C#. It enables direct database access over a relational vendor-independent data layer as well as automated persistence through O/R mapping containers that work upon the data layer themselves.

AtomsFramework
The JC Framework and AtomsFramework Projects provide an Object/Relational Persistence Layer for VB6 and Visual Studio .NET (sometimes called an O/R Mapper or an O/R Layer). The project is similar in concept to Microsoft's ObjectSpaces but is not tied to any particular database and best of all you don't need to wait for "Whidbey" or "Yukon" to be able to use it.

Bamboo.Prevalence
Bamboo.Prevalence is a .NET implementation of the object prevalence concept brought to life by Klaus Wuestefeld in Prevayler. Bamboo.Prevalence provides transparent object persistence to deterministic systems targeting the CLR.

BBA DataObjects
BBA DataObjects is an object-to-relational mapping library for Microsoft .NET that maps SQL Server tables to Objects, so you can build real Object Oriented Applications in a fast and easy way. Top features: Lazy Load, Paged Loading, Cryptography.

csopf
The C# Object Persistent Framework (csopf) is a project which has a goal of making development of business software as rapid and as maintainable as possible. The philosophy is to create a framework which is very developer oriented, to create clean, obvious and readable code. It also lets the developer develop software, with database administration handled automatically.

Gentle.NET
Gentle.NET is an RDBMS independent object persistence framework. It features automatic SQL generation and object construction, an SQL factory for creating custom queries, DataView construction helpers, excellent performance and reasonably complete docs.

iBATIS.NET DataMapper
The iBATIS framework makes it easier to use a database with Java and .NET applications. iBATIS couples objects with stored procedures or SQL statements using a XML descriptor.

NHibernate
NHibernate is a .NET based object persistence library for relational databases. NHibernate is a port of the excellent Java Hibernate relational persistence tool.

NPersist
The NPersist Framework is a Free, Open Source (LGPL) .NET O/R Mapping Persistence framework capable of persisting standard .NET Objects (POCO Persistence) to one or several relational databases using Object/Relational mapping information provided in an XML file.

ObjectBroker
ObjectBroker is an object relationnal mapping framewok for the .NET plateform. It provide transparent persistance and transaction management.