from qgis.core import *

suffix = "_foo"
pathToFile = "/path/to/wherever/"

layers = iface.legendInterface().layers()
for layer in layers:
    newName = layer.name() + suffix + ".shp"
    ret = QgsVectorFileWriter.writeAsVectorFormat(layer, pathToFile + newName, "utf-8", None, "ESRI Shapefile")
    if ret == QgsVectorFileWriter.NoError:
        print newName + " saved to " + pathToFile + "!"

For Raster Layer use QgsRasterFileWriter

€: You can find a more detailed example in the Cookbook.

from qgis.core import *

suffix = "_foo"
pathToFile = "/path/to/wherever/"

layers = iface.legendInterface().layers()

for layer in layers:
    newName = layer.name() + suffix + ".shp"
    ret = QgsVectorFileWriter.writeAsVectorFormat(layer, pathToFile + newName, "utf-8", None, "ESRI Shapefile")
    if ret == QgsVectorFileWriter.NoError:
        print newName + " saved to " + pathToFile + "!"

For Raster Layer use the QgsRasterFileWriter

P.S.: You can find a more detailed example in the PyQGIS Developer Cookbook.

from qgis.core import *

suffix = "_foo"
pathToFile = "/path/to/wherever/"

layers = iface.legendInterface().layers()
for layer in layers:
    newName = layer.name() + suffix + ".shp"
    ret = QgsVectorFileWriter.writeAsVectorFormat(layer, pathToFile + newName, "utf-8", None, "ESRI Shapefile")
    if ret == QgsVectorFileWriter.NoError:
        print newName + " saved to " + pathToFile + "!"

For Raster Layer use QgsRasterFileWriter

from qgis.core import *

suffix = "_foo"
pathToFile = "/path/to/wherever/"

layers = iface.legendInterface().layers()
for layer in layers:
    newName = layer.name() + suffix + ".shp"
    ret = QgsVectorFileWriter.writeAsVectorFormat(layer, pathToFile + newName, "utf-8", None, "ESRI Shapefile")
    if ret == QgsVectorFileWriter.NoError:
        print newName + " saved to " + pathToFile + "!"

For Raster Layer use QgsRasterFileWriter

€: You can find a more detailed example in the Cookbook.