Dunno about "most used", but certainly "a relief that it is extremely useful" as it was outrageously expensive, but we put a boiling tap in when we redid the kitchen. I work from home, and have a couple of co-workers here during the day, and time not spent standing waiting for the kettle (or re-boiling it having walked away - for too long!) is much appreciated, saves time filling a saucepan with boiling water, and when we forget to pre-warm plates for the meal we can just run them under the boiling tap briefly.
I like my Soup Maker - basically a liquidiser with a heating element - which I use to make a bowl of soup out of whatever leftover veg was over-catered for, or left on the side of a plate, the night before. Chuck in some stock / cube and a chilli maybe, press the button and soup is ready half and hour later when it beeps. In Asparagus season I use it on all the woody stalks that would otherwise be thrown away, ditto for Broccoli stalks etc. (I put the soup through a Mouli - another essential gadget! - to get rid of the stringy bits)