AssetDatabase.GetSubFolders

Description

Given a path to a directory in the Assets folder, relative to the project folder, this method will return an array of all its subdirectories.

using UnityEditor;
using UnityEngine;

public class AssetDatabaseExamples : MonoBehaviour
{
    [MenuItem("AssetDatabase/Get Folder List")]
    static void SubFolderExample()
    {
        //This method prints out the entire folder list of a project into the console
        var folders = AssetDatabase.GetSubFolders("Assets");
        foreach (var folder in folders)
        {
            Recursive(folder);
        }
    }

    static void Recursive(string folder)
    {
        Debug.Log(folder);
        var folders = AssetDatabase.GetSubFolders(folder);
        foreach (var fld in folders)
        {
            Recursive(fld);
        }
    }
}