Math stuff

• Numeral systems/Number representations: convert between various bases, Roman numerals, scientific notation, prime factorization, etc. (requires JavaScript)
• Number types calculator (perform calculations with complex numbers, quaternions, various types of infinity, p-adic numbers, etc.) (requires JavaScript)
• Unit converter (length, time, etc.) (requires JavaScript)
• Vector calculator (requires JavaScript)
• Function calculator (requires JavaScript)
• "Everything" formula (a.k.a. Tupper's self-referential formula) (requires JavaScript)
• Latitude/longitude / spherical coordinate distance (only an approximation, because the Earth is only approximately a sphere) (requires JavaScript)
• Kaufman decimal sorter: a generalization of repeated decimals (like 0.3333333…) from Math with Bad Drawings (requires JavaScript)
• Aztec diamond tiler: generates domino tilings of Aztec diamonds, as described in a Mathologer video (requires JavaScript)
• Number of ways of making change: based on a recent Mathologer video, this shows the number of ways of making change for a given amount (requires JavaScript)

Date and time stuff

• Time zone converter (requires JavaScript)
• Date representation converter (requires JavaScript)
• Date anagrams: find dates like 2017-02-17 or 2017-07-12 where the date is the anagram of the year
• Another idea for abolishing time zones (which I don't expect to actually catch on)
• How to find the number of Friday the 13ths in a given year

See also

• Math Finger Paintings, my final project in high school calculus (List)
• Yet another Monty Hall problem demo and Reverse Monty Hall (in Games) (requires JavaScript)