This little script will import all the assets contained in the folder you currently have selected, and all it's sub-folders. Just save this script in a folder in your project called Editor. Next, go to Assets > Reimport Folder and it will reimport the folder you currently have selected.

import System.IO;

@MenuItem( "Assets/Reimport Folder" )

static function ImportFolder()
{
    var realPath : String = Application.dataPath;
    realPath = realPath.Remove( realPath.Length - 6 ); 
    var selectedPath : String = realPath + AssetDatabase.GetAssetPath( Selection.activeObject );   
    var fileEntries : String[] = Directory.GetFiles( selectedPath, "*", SearchOption.AllDirectories );
    for( var file : String in fileEntries )
    {
       file = file.Replace( "\\", "/" );
       file = file.Remove( 0, realPath.Length );
       AssetDatabase.ImportAsset( file );
    }
}

i needed this to reload screenshots that i use as savegame thumbnails. just saying that this did the job for me:

AssetDatabase.Refresh();

I still don't get it. You copy the script in a txt flile, name it randomly and place it inside Editor folder in Unity installation dirrectory? It doesn't work for me. I hope that this script would solve my problem: when I import a big pack of assets linked together(by copying all the folders of the pack inside Assets folder), when Unity imports all, it crashes. If I import only parts of assets, all the links are broken.

Why would you need to reimport?

Any directories within the resource folder can be accessed with Resources.Load() and obviously if you overwrite a resource, which you have pointed to in your could it will be automatically "reimported" anyway. If this isn't what you looking for, comment back :)