NHibernate is not really a good fit for our environment due to all the dependencies. (Castle, log4net etc.)
Is there a good lightweight alternative?
Support
Generally speaking, for your database backend to work with .net you need an ADO.Net provider for it.
For MS Access (Jet), the Provider is shipped with .net. For SQLite, there is a selfcontained ADO.Net Provider.
As for the data access layer lib, if you want some abstraction over ADO.Net:
All those work well starting with framework 2.0 and up.
Basically, you choose (and there is a lot of choices)