Probably a silly question, because I assume the N stands for ".NET", though I could be wrong. I'm interested in where this naming convention got started.
Examples:
NHibernate
NLog
NCover
NDepend
Ninject (I realize this is supposed to be a play on "ninja", but it still fits)
The same reason why Apple uses "i".
Brand recognition.
You immediately know it's a Microsoft .NET project.
usually it means that original project (without N) was done for dome different language/framework. F. e. NHibernate was inspired by Hibernate for Java. So N means .net. For NUnit there is a whole family, like pyUnit, JUnit, etc.