While I was trying to find the answer online, I stumbled upon this answer from Quora. So in case of nVidia, first letter "n" means a pronumeral (i.e. variable). I was wondering, if it's the case for other names such as Nhibernate, NSubstitute, etc. Does it mean something different?
In the case of nvidia is just a play on the letters/words. nVidia sounds like 'invidia' which is latin for envy. In the old days of Rome, it was said that those with envy had green eyes (hence the nvidia logo).
The 'n' prefixing different libraries stands for (in most cases) that is for .net framework.
One classic example of this is nUnit (a unit testing framework for .net). There are multiple libraries for other frameworks/runtimes similar to this one line xUnit (also .net), jUnit (Java) or RunIt (Ruby).
nHibernate is the .net implementation of Hibernate (ORM library on java) and so on.