The TerrainData plugin can export and import.
dae (Collada) files, also supporting rigged SkinnedMeshRenderers. m4a files from WebGL builds and Unity 4 sound files. wav files from U5's AudioClip assets using FMOD. tga files and decode&encode most texture formats used by Unity.
There are multiple plugins to convert Unity assets from/to common file formats : Custom MonoBehaviour types also are supported. Type information extracted from Unity is used in order to generate text representations of various asset types. It can create standalone mod installers from changes to.
