Does anyone know of something similar for Windows?
DevUtils has a comparison page at [1] describing the differences between both tools. To their credit (I reckon it is rare on such "comparison" pages), they even highlight the features of their competitor that they don't match.
I regularly use the JSTool (for Javascript/JSON), MIME tools (for Base64 and URL encode/decode) and Markdown++ plugins. There is also a NppCalc calculator app.