For a project which controls a mains powered appliance, I think it would be nice to have a complete, enclosed unit consisting of a mains to 3.3v/5.0v converter, relay, fuse if deemed necessary, and a connector block.
This would keep the high voltage stuff safely isolated and allow a more compact overall design.
I’ve put together a time switch recently and had problems trying to keep the whole thing to a reasonable size, while respecting safety and isolation between the high and low voltage side.
All I managed to find at a reasonable price was an enclosed power supply http://www.banggood.com/AC-DC-Isolated-AC-110V-220V-To-DC-5V-600mA-Constant-Voltage-Switch-Power-Supply-Converter-Module-p-1088877.html?rmmds=search
Here is what I put together using individual components. The power supply (the green board) is 2A which is more than is required (dimensions 66mm X 31mm) but I could imagine one nice little plastic box (or maybe one board) including all those components together would be generally useful for mains power projects and allow a smaller overall design.