There are many different OR mappers available.  The three products that I consider when thinking about the object relational mapping category are NHibernate, LLBL Gen Pro, RapTier and Wilson ORM.

NHibernate is the open source port of the popular OR mapper Hibernate for java.  I am not a java developer so I can not speak to how good of a port it is.  I have not used NHibernate myself, mainly because I have found the documentation of the product to be poor so far.  There is a very active community around this product, but for my project I did not want to participate in their project, I wanted to focus on building mine.  I have read and heard of success stories from colleagues along with their accompanying horror stories with NHibernate.  I do consider NHibernate to be a true mapper because it does not generate code for the developer to manage.  It does all its work dynamically once configured.

The second mapper is LLBL Gen Pro, which comes with its own environment for mapping objects with database tables and fields.  This product does generate code files, so it is not something that I wanted to use this time around.

The third product is called RapTier.  I have used this on a couple different projects earlier this millennium with success.  RapTier does create code files though, so you have to maintain those and is something I would not consider doing now.  But at the time, I experienced this product to be a great time saver because it allowed me to focus on the application and not the data access. The company also provided good support and an aggressive price. 

The last product is WilsonORMapper and is ultimately what I decided to use on this project.