In QGIS 2.18 there was plugin "Copy layers and groups to clipboard". It was allowing just in two clicks to copy/paste layer(s) with style and etc. from one project to another.
Is there something similar in QGIS 3.0?
Animation below shows the plugin in action:
Best Answer
This functionality has been added out-of-the-box for QGIS 3.2. You can now right click a layer or group and select "Copy Layer" (or "Copy Group"), and then right click in the legend of another QGIS window and select Paste Layer/Group: