Cut&Slice me is a free Photoshop plugin that makes precise slices & exports your layers.
Cut&Slice me cut and exports your layers in PNG format trimming the pixels you don´t need, or you can specify the size you want, or you can export all states of your buttons in the blink of an eye.
Cut&Slice me export to iPhone and Android, also removes all unsupported characters in the file name. For iPhone design for retina, for Android design for xhdpi and then export to ldpi, mdpi and hdpi with only one click.