Lightweight alternatives to NHibernate

Massive - https://github.com/robconery/massive

or

PetaPoco - https://github.com/toptensoftware/petapoco

Both are a single .cs file with no dependencies except what's in the GAC.

(full disclosure, PetaPoco is something I wrote)

For a lightweight ORM that performs well and only requires a single assembly why not try out Lightspeed from Mindscape. It's not open-source, however source is available and it's reasonably priced - the risk with most ORM's that aren't well adopted is of course quality and level of support, and there are very few other open source ORM's worth bothering with in the .Net space at the moment.

Because of your dislike of NHibernate's dependencies it sounds like you don't have a need for a logging framework or any of the castle project facets i.e. IoC, Monorail etc. Have you considered maybe just taking the bare minimum of NHibernate requirements (log4net and the Iesi collections I believe, and dynamic proxy from the castle project?) and running ILMerge over them to consolidate them into a single assembly - might take a bit of fiddling, but it's not too hard - or alternatively you could just pull the source code for each of these projects into a custom build of NHibernate you maintain for your organization that trims out the features not required by your project/organization - it's not as hard/akward as it sounds and I've done something along these lines for one project where we wanted to benefit of an ORM, but needed to reduce the size of the distributed files/installer.

Also - are you perhaps able to explain what you feel is too "heavy" about an Nhibernate based solution ... in my experience its a reasonably lightweight ORM framework compared to some.