Exports a mesh instance from the currently selected Mesh Filter. Should go in the Editor folder. http://blog.becksebenius.com/ for more.

MeshExporter.cs

using UnityEngine;
using UnityEditor;

public class MeshExporter
{
    const string menuItemPath = "Assets/Export Mesh Instance";
	
	[MenuItem(menuItemPath)]
	public static void SaveMeshAsAsset ()
	{		
		var go = Selection.activeGameObject;
		if(!go)
		{
			EditorUtility.DisplayDialog("Invalid Selection", "You must select a game object with a mesh filter.", "Confirm");
			return;
		}
		
		var mf = go.GetComponent<MeshFilter>();
		if(!mf)
		{
			EditorUtility.DisplayDialog("Invalid Selection", "You must select a game object with a mesh filter.", "Confirm");
			return;
		}
		
		var mesh = mf.sharedMesh;
		if(!mesh)
		{
			EditorUtility.DisplayDialog("Invalid Selection", "Selected mesh filter does not have a mesh applied.", "Confirm");
			return;
		}
		
		string path = EditorUtility.SaveFilePanelInProject("Cache Mesh", "mesh", "asset", "Pick a location to cache your mesh.");
		if(path == null) return;
		if(path == "") return;
		
		var obj = AssetDatabase.LoadAssetAtPath(path, typeof(UnityEngine.Object));
		if(obj)
		{
			if(!EditorUtility.DisplayDialog("Overwrite asset?", "An asset already exists at this path. Overwrite?", "Confirm", "Cancel"))
			{
				return;
			}
			
			if(!(obj is Mesh))
			{
				if(!EditorUtility.DisplayDialog("Destroy asset?", "The path selected contains a non-mesh asset. Destroy the original asset file?", "Confirm", "Cancel"))
				{
					return;
				}
			}
		}
		
		if(EditorUtility.IsPersistent(mesh))
		{
			mesh = GameObject.Instantiate(mesh) as Mesh;
			mf.sharedMesh = mesh;
		}
		
		AssetDatabase.CreateAsset(mesh, path);
	}
}