Ayende has a recent post talking about Orthogonal Frameworks in relation to open source projects.
Personally, being a developer for an educational institution (a public school district), this is the reason open source frameworks so appealing. The last thing I want to do is require the district to purchase a framework that is closed source. This creates more problems than it solves. I won't get on the soapbox and preach about why I like using open source projects so much, but it makes all the sense in the world for educational institutions.
If I ever want to modify or drive the development into a different direction, I don't have to ask for more funds from my directors, I simply extend the functionality that I am looking for. This in itself, is priceless.
There is a few side effects to using open source projects, the main one being it takes a good level of understanding as most open source projects are trickier to use than commercial products, and sometimes the documentation is lacking.
This brings me to my final point about NHibernate. When I first started looking at O/R Mappers for the district, I was very turned off by the majority of the commercial frameworks for the reason that Ayende states, they make alot of assumptions on how their framework is going to be used which makes for more difficult usage of the mapper. Since NHibernate is open source, if I ever do needed additional functionality (which I haven't yet), its available to me. In addition, NHibernate has a very large following and support staff which is only growing every day. The Castle Project is another framework that this applies to as well, and I have begun to use in my day to day development of web projects.