NBuilder is a very good fluent-API library for generating data. It uses rules that you define and isn't "random" per se. You may be able to randomize the inputs to the API, though, to suit your needs.
Since this still gets some attention I think it's worth mentioning the project is now available through NuGet (https://www.nuget.org/packages/NBuilder/) as well, though it hasn't been modified since 2011.
