iSlice

iSlice can open PSD documents retaining layers and layer comps. Hence you can easily switch between various layer comps in the document and also hide/show layers.
You can mark a rectangular portion of the document for export.

Since iSlice retains the layer data, user may even select a set of layers and export it, ignoring the background layers.
iSlice gives live preview of the optimized image along with the expected size of the optimized image so that you can balance between quality and size of the output.