Python. Most popular language for data science in industry. Also well-suited to application building.
R. Most popular language among statisticians. Excellent focus on user experience, and lots of amazing tooling.
Julia. A beautifully designed, fast, and increasingly popular language for data science and other scientific computing applications. My personal favorite.
Plotly. Beautiful, easy-to-make graphs for the web (both 2D and 3D). Very well integrated with Python, R, and Julia.
Tidyverse. An opinionated but powerful collection of packages for doing data science in R.
Esquisse. An R package for creating data visualizations quickly and beautifully, without code.
Spark. Distributed computing framework, commonly used in industry.
Jupyter. Very popular notebook computing environment which supports many languages, including Julia, Python, and R.
Pluto. Julia-specific alternative to Jupyter which is reactive (changing anything causes everything downstream of it to change too).
RStudio. Premier development environment for R. Can also be used for both Python and Julia.
Superset. Open source data visualization dashboards.
RAWGraphs (rawgraphs.io) Kind of like Esquisse (or Superset), but it works as a web app that you can paste your data into as a CSV. Gives you a ton of cool visualizations and ways to map the data.
CoCalc. Cloud environment with excellent collaboration tools which supports practically every piece of open source scientific computing software.
Colab. A Google-managed cloud Jupyter environment; Python-only and with slightly hackish support for collaboration, but it does include free GPU, which is important for deep learning and some other applications.
NextJournal. Another free-GPU cloud Jupyter service.
Introduction to Data Science in R. Superb online book written by Rafael Irizarry at Harvard.
Kaggle. Datasets, problems, and contests.
Content creation tools and utilities
Prismia. Create and deploy interactive lessons.
Desmos. A mathlet creation environment for everyone.
math3d.org. Desmos, but in 3D. Really beautiful.
JSXGraph. Create reactive mathlets (with code). What Prismia uses for its 2D mathlet system.
Ipe. Desktop application for making precise 2D mathematical figures quickly (using a graphical user interface).
TinyTeX. An easy-to-use distribution of LaTeX that does not take up a ton of hard drive space.
MathPix. Snapshot math expressions or tables on your screen and convert them automatically to LaTeX (using optical character recognition).
TextSniper. Like MathPix, but with plain text. For macOS.
Keyboard Maestro. Automate tasks at the operating system level (e.g., scroll down 60 pixels and click, then repeat 100 times). Hopefully you don't need it often, but it's a lifesaver when you do.
Copier. Templating system for quickly creating new files or projects on your computer (a Python package). Especially useful for software projects with boilerplate, stock letters, etc.
ungit. A visual approach to version control. Solves a lot of Git's UX problems.
Svelte. A front-end framework that's simple, really fast, and genuinely reactive.
Visual Studio Code. The development environment that all the cool kids are using these days.
Firebase. An easy way to create a low-cost, scalable web application. Hosting, database, and cloud functions are all integrated and well documented.
AG Grid. Possibly the best open source data table package for the web. Highly recommended over plain HTML for large tables.
Art of Problem Solving. Premier math learning community for middle school and high school.
Mathigon. Most polished, beautiful math lessons on the internet.
Data Gymnasia. Data science content built on Mathigon technology.
3Blue1Brown. The best math videos on YouTube. Beautiful visualizations, original exposition, and excellent storytelling.
Distill.pub. Visually stunning interactive articles on data science topics.
Concrete Mathematics by Knuth, Graham, and Patashnik. Original, expertly crafted book on a variety of discrete math topics.
Probability with Martingales by David Williams. Succinct probability textbook from the University of Cambridge (the one I learned basic probability from).
Linear Algebra Done Right by Sheldon Axler. An expository masterpiece with an original and elegant take on basic linear algebra.
Visual Complex Analysis by Tristan Needham. The title says it all.
Bike Friday. Really cool folding bicycles (better ride).
Brompton. Really cool folding bicycles (more compact).
Taza Chocolate. Amazing chocolate made locally in Somerville. Unique, slightly gritty texture (and dairy free).
xckd. Premier nerd comic on the web.
Quora. Social media site built around the question-and-answer concept. Lots of excellent writers and a good algorithm for surfacing quality content.
Meetup.com. Find folks in your area with common interests.
Pocket. Browser extension and app for stashing articles you find and want to read later.