Blender add-on for automatic unique output path generation based on the current timestamp

auto_filepath.py

bl_info = {
    "name": "Auto-Filepath",
    "author": "Robert Guetzkow",
    "version": (1, 0, 2),
    "blender": (2, 80, 0),
    "location": "Output Properties > Auto-filepath",
    "description": "Automatically sets a unique filepath for each frame based on the current timestamp.",
    "warning": "",
    "wiki_url": "",
    "category": "Render"}

import bpy
import datetime
from pathlib import Path
from bpy.app.handlers import persistent


@persistent
def update_filepath(self):
    if not bpy.context.scene.auto_filepath_settings.use_auto_filepath:
        return
    now = datetime.datetime.now()
    path = Path(bpy.context.scene.auto_filepath_settings.directory)
    base_path = path / now.strftime('%Y-%m-%d_%H_%M_%S_%f')
    bpy.context.scene.render.filepath = str(base_path / "####")
    bpy.context.scene.render.use_file_extension = True
    bpy.context.scene.render.use_overwrite = False
    if bpy.context.scene.use_nodes:
        for node in bpy.context.scene.node_tree.nodes:
            if node.type == "OUTPUT_FILE":
                node.base_path = str(base_path / node.name)


def set_directory(self, value):
    path = Path(value)
    if path.is_dir():
        self["directory"] = value


def get_directory(self):
    return self.get("directory", bpy.context.scene.auto_filepath_settings.bl_rna.properties["directory"].default)


class AutoFilepathSettings(bpy.types.PropertyGroup):
    use_auto_filepath: bpy.props.BoolProperty(name="Automatic filepath generation.",
                                              description="Enable/disable automatic filepath generation. When enabled, "
                                                          "this will overwrite the output path and the base path of "
                                                          "all File Output nodes.",
                                              default=False)

    directory: bpy.props.StringProperty(name="Directory",
                                        description="Directory where files shall be stored.",
                                        default="/",
                                        maxlen=4096,
                                        subtype="DIR_PATH",
                                        set=set_directory,
                                        get=get_directory)


class AUTOFILEPATH_PT_panel(bpy.types.Panel):
    bl_label = "Auto-Filepath"
    bl_space_type = "PROPERTIES"
    bl_region_type = "WINDOW"
    bl_context = "output"
    bl_options = {"DEFAULT_CLOSED"}

    def draw_header(self, context):
        self.layout.prop(context.scene.auto_filepath_settings, "use_auto_filepath", text="")

    def draw(self, context):
        layout = self.layout
        layout.prop(context.scene.auto_filepath_settings, "directory")


classes = (AutoFilepathSettings, AUTOFILEPATH_PT_panel)


def register():
    for cls in classes:
        bpy.utils.register_class(cls)
    bpy.types.Scene.auto_filepath_settings = bpy.props.PointerProperty(type=AutoFilepathSettings)
    if update_filepath not in bpy.app.handlers.render_pre:
        bpy.app.handlers.render_pre.append(update_filepath)


def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)
    del bpy.types.Scene.auto_filepath_settings
    if update_filepath in bpy.app.handlers.render_pre:
        bpy.app.handlers.render_pre.remove(update_filepath)


if __name__ == "__main__":
    register()

Here is my take on this. Rather than generating new folder for each image I just save images with timestamps in filename.
https://gist.github.com/QuietNoise/c56c5f4bf1e42238928bf2cef481cde0

@QuietNoise I think I originally wrote this for animations, hence the separate folder. Thanks for leaving a link to the adapted version, might be useful for someone looking at the old question on Stack Exchange.