The MemoryLayerSaver plugin automatically saves "memory" provider layers as GML files alongside the project file.
Several other plugins (eg the contour plugin) create map layers for which the data is held in memory rather than in persistent storage. When the project is saved the definition of the layer is stored, but the content is not saved. This plugin automatically saves all such layers as GML files alongside the QGIS project file so that when the project is reloaded the memory provider layers are intact.
Manager: Chris Crook