UVPackmaster is a free add-on for Blender (you can download the latest version from the Downloads page, the name of the add-on zip ends with blend2.8x). You can install it using the standard procedure for Blender add-ons.
The main functionality of the add-on is to enable UVP engine packing operations in Blender. Note that you still need to purchase a license for the engine itself in order to use packing functions - the add-on is only a driver for the engine, which must be installed as a separate component. To learn more about the UVP engine read this page.
Apart from that there are some basic functionalities which are provided by the add-on on its own (the engine is not required) - they are available for free for everyone. Read below in order to learn more about them.
Split Overlapping Islands / Undo Island Split
When you press the Split Overlapping Islands button, the add-on will move all overlapping islands among the selected islands to the subsequent UV tiles (in the X axis direction), so that they are not overlapping anymore. After the split operation you can press Undo Island Split - the selected islands will be moved back to their original locations.
Keep in mind that the undo operation depends on the UV layout state after splitting. So if you manually move an island after it was split, then the undo operation won’t be able to find its original location. The undo operation may also fail, if you manually join two different islands after splitting.
Adjust Scale To Unselected
This operation will independently resize every selected island, so it has a coherent scale with the islands which are not selected. If you do not want an island to take part in the operation, then you should hide it.
Technically this operator works as follows: for every unselected island it computes a ratio: 3D area / UV area. Afterwards it calculates the average ratio of all unselected islands. Eventually it independently scales every selected island, so the resulting 3D area / UV area ratio of the island is equal to the previously computed average ratio of the unselected islands. This approach is similar to the Blender built-in Average Island Scale operator, the difference is that the Blender operator unifies the ratio among selected islands, while the Adjust Scale To Unselected operator transfers the ratio of the unselected islands to the selected islands.
If an island is degenerated (has 3D area or UV area very close to 0), then it will be silently ignored during the operation.