http://www.plottingtool.com/app.htmlIt's a fully client-side, Tableau-inspired data visualization & exploration tool for CSV / tabular data. I built it around 7 years ago with just ES6 and d3.js (no React etc) but never really finished or "launched" it - perfectionism led to a neverending todo list, and with a target market of only myself I didn't have a strong pull toward solving a real-world use case. I even took some time between jobs thinking I'd build it into a real product, but only managed to burn myself out instead (quite a surprise to learn that could happen on a passion project!)
But it does work - drag in a CSV file (ideally <100MB) and you can make some cool charts and quickly explore a dataset, all without the data leaving your machine. The features aren't very discoverable, but it has stuff like adding derived columns, various fancy charts, row/column sorting, custom colors, etc. I'm sure there are far better tools around these days, but I've still come back to this thing a surprising amount in my day job when I want to whip up a quick histogram or investigate some trend in a medium-size dataset.