Everyone knows that plugins are only supported in Unity Pro. It's also a common knowledge here (on Forums and Answers) that if you have a native dll, you have to put it in Plugins folder and own the Pro license to be able to use it. Actually this knowledge is wrong, at least on Windows. The documentation states that plugins are native libraries, but it doesn't state that every native library is a plugin. So, you can call...
Unity free, plugins and native DLLs
Unity free, plugins and native DLLs