Adds turntable rotation that rotates freely independently of the view rotation, with out gimbal lock.

space_view3d_trackball_all_axes.py

bl_info = {
    'name': 'Rotate trackball in all axis',
    'author': '',
    'version': (0, 0, 1),
    'blender': (2, 6, 7),
    'location': '3d view > Alt + LMB-drag',
    'description': 'Add trackball rotation that rotates freely around all axes.',
    'wiki_url': '',
    'tracker_url': '',
    'category': '3D View'}

import bpy
from mathutils import *
from math import *

class RotateTrackballAllAxes(bpy.types.Operator):
    '''Trackball rotation in all axes.'''
    bl_idname = "view3d.trackball_all_axes"
    bl_label = "Trackball rotation in all axes"
    
    angle_yaw=0
    angle_pitch=0
    mouse_last_pos=Vector((0,0))
    
    def execute(self, context):
        region_3d = context.space_data.region_3d
        
        xa=Vector((1,0,0))
        ya=Vector((0,1,0))

        # calculates rotation in the camera space
        camera_rotation_pitch=Quaternion(xa,self.angle_pitch)
        camera_rotation_yaw=Quaternion(ya,self.angle_yaw)
  	
        selected_objects = len(bpy.context.selected_objects) > 0 and bpy.context.user_preferences.view.use_rotate_around_active

        if selected_objects:
            '''
                calculates view_location rotation in world space to counteract
                the pivot rotating around keeping it in it's current position
            '''
            saved_location = bpy.context.scene.cursor_location.copy()
            bpy.ops.view3d.snap_cursor_to_selected()
            pivot_location = bpy.context.scene.cursor_location.copy()
            bpy.context.scene.cursor_location = saved_location

            xa.rotate(region_3d.view_rotation)
            ya.rotate(region_3d.view_rotation)

            world_rotation_pitch=Quaternion(xa,self.angle_pitch)
            world_rotation_yaw=Quaternion(ya,self.angle_yaw)

            pivot_to_camera = region_3d.view_location - pivot_location
            pivot_to_camera.rotate(world_rotation_yaw*world_rotation_pitch)

            region_3d.view_location = pivot_location + pivot_to_camera


        region_3d.view_rotation=region_3d.view_rotation*camera_rotation_yaw*camera_rotation_pitch

        return {'FINISHED'}
    
    def modal(self, context, event):
        region_3d = context.space_data.region_3d
        
        if event.type == 'MOUSEMOVE':
            #find the yaw and pitch angles from the mouse position variation
            mouse_pos=Vector((event.mouse_region_x,event.mouse_region_y))
            self.angle_yaw=-(mouse_pos.x-self.mouse_last_pos.x)/200.0
            self.angle_pitch=(mouse_pos.y-self.mouse_last_pos.y)/200.0
            self.execute(context)
            self.mouse_last_pos=Vector((event.mouse_region_x,event.mouse_region_y))
        
        elif event.type in {'LEFTMOUSE', 'MIDDLEMOUSE', 'RIGHTMOUSE', 'ESC'}:
            return {'FINISHED'}
        
        return {'RUNNING_MODAL'}
    
    def invoke(self, context, event):
        
        if context.space_data.type == 'VIEW_3D':
            region_3d = context.space_data.region_3d
            self.mouse_last_pos=Vector((event.mouse_region_x,event.mouse_region_y))
                        
            context.window_manager.modal_handler_add(self)
            
            if region_3d.view_perspective == 'CAMERA':
                region_3d.view_perspective = 'PERSP'
            
            return {'RUNNING_MODAL'}
        
        else:
            return {'CANCELLED'}


def register():
    bpy.utils.register_class(RotateTrackballAllAxes)
    
    wm = bpy.context.window_manager
    km = wm.keyconfigs.addon.keymaps.new(name="3D View", space_type='VIEW_3D')
    kmi = km.keymap_items.new('view3d.trackball_all_axes', 'EVT_TWEAK_L', 'ANY', shift=False, ctrl=False, alt=True)


def unregister():
    bpy.utils.unregister_class(RotateTrackballAllAxes)
    
    wm = bpy.context.window_manager
    km = wm.keyconfigs.addon.keymaps['3D View']
    for kmi in km.keymap_items:
        if kmi.idname == 'view3d.trackball_all_axes':
            km.keymap_items.remove(kmi)

if __name__ == "__main__":
    register()