Potato ricers are great. A single task tool, but one of the most effective there is. A thermometer like a Thermapen isn't cheap as a headline figure, but the cost per use is basically negligible over its life, so that'd be right up there. Tongs have been mentioned, but I like the ones with silicone pincers because you don't have to worry about any non-stick pan you're using, etc.
The above might be tools rather than gadgets, but meh!