Cool Websites — September 18, 2026

33 corners of the internet worth bookmarking this week, from a DNS propagation checker to a site where you bounce cats.

🔐 Security & Privacy

Mozilla Monitor

Scans known data breaches for your email address and tells you which ones leaked it, with concrete steps to lock down each account. No sign-up needed to run a scan.

Privacy.com

Generates single-use or merchant-locked virtual card numbers so a sketchy checkout page never touches your real card. US-only, but the free tier covers most everyday shopping.

Guerrilla Mail

Spins up a disposable inbox in seconds for sign-up forms you don't trust. Messages self-destruct after an hour, and you can reply from the address if you need to.

Startpage

Routes your query through Google, strips out every tracker and cookie, and hands you back Google-quality results without building a profile on you in the process.

🕵️ OSINT & Investigation

IntelTechniques

Michael Bazzell's free OSINT tool index: hundreds of hand-picked lookup tools sorted by category (usernames, phone numbers, images) instead of one search box claiming to do everything.

Social Blade

Tracks growth history for YouTube, Twitch, TikTok, and Instagram accounts going back years, useful for spotting a fake follower spike or sizing up a channel before a sponsorship deal.

CachedView

Pulls a page's cached or archived version from the Wayback Machine, archive.today, and a couple of other sources in one search. Handy when a site goes down or quietly edits a page.

Onyphe

Runs weekly scans across the entire internet and lets you search the results, an alternative to Shodan and Censys with its own dataset of exposed devices and services.

🧑‍💻 Cyber / Hacking Tools

VulnHub

Hosts intentionally broken virtual machines you download and try to break into yourself, a hands-on alternative to reading about penetration testing instead of doing it.

Blue Team Labs Online

Flips the CTF format around: instead of breaking into systems, you work incident-response scenarios and forensic evidence like an actual SOC analyst on shift.

Exploit-DB

OffSec's searchable archive of public exploits and proof-of-concept code, the reference researchers pull up to check whether a CVE actually has working exploit code attached.

AbuseIPDB

A crowdsourced blacklist where sysadmins report IPs caught spamming, scanning, or brute-forcing logins, so you can check an address's rap sheet before deciding to block it.

🖥️ IT & Sysadmin Tools

tldr pages

Rewrites man pages as short, example-driven cheat sheets. Type tldr tar and get five commands people actually use instead of a wall of flags nobody memorizes.

WebPageTest

Runs your site through real browsers on real devices from 40 locations and returns a waterfall chart of exactly what loaded slowly and why. Open-source and free, where several rivals now paywall the useful parts.

whatsmydns.net

Checks a DNS record against resolvers in 20-plus countries at once, so after you change a record you can watch how far the update has actually spread instead of guessing and refreshing.

🤖 AI Tools

You.com

An AI search layer that cites its sources inline and switches between chat, code, and image modes without ever leaving the results page.

HuggingChat

Free chat interface for open-source models, currently routing across more than a hundred of them, with an auto mode that picks whichever model actually fits your question.

Udio

Type a genre and a lyric idea, get back a full song with vocals in under a minute, a different flavor of AI music generation than the more famous name in this space.

Julius AI

Upload a spreadsheet, ask questions about it in plain English, and it writes and runs the analysis code itself, then hands back a chart instead of a formula you'd have to build.

🎬 Video & Image Tools

Descript

Edits video and audio by editing the transcript: delete a sentence in the text and the matching clip disappears with it. The free plan covers recording, editing, and export.

Luma AI

Turns a text prompt or still photo into a short, physically plausible video clip in under a minute. Free generations are watermarked; clean exports start at $30 a month.

ImgBB

No-frills image host: drop a file in, get a direct link and BBCode back immediately, no account required for a quick upload.

🛠️ Developer Tools

draw.io

Free diagramming tool for flowcharts, ER diagrams, and network maps that imports files straight from Visio or Lucidchart if you're trying to leave either one behind.

Can I Use

Look up any CSS or HTML feature and see exactly which browser versions support it, the tab every front-end developer keeps open somewhere in the background.

Swagger Editor

Write an OpenAPI spec on the left, watch the rendered API docs update live on the right, and catch formatting mistakes before they reach your real documentation.

💰 Money & Finance

Bogleheads Wiki

Over a thousand pages of investing reference written and maintained by an online community built around low-cost index investing. No ads, no affiliate links, no login wall.

Portfolio Charts

Backtests any asset allocation against 56 years of market data and shows the worst-case drawdown, not just the rosy average return everyone likes to quote.

🎮 Games

AI Dungeon

An open-ended text adventure where an AI improvises the story around whatever you type next. No two playthroughs land the same way.

Diep.io

Multiplayer tank arena where you grind shapes for XP and pick an upgrade path at every level, from the creator of Agar.io.

📚 Learning & Reference

The Feynman Lectures on Physics

Caltech's free, fully rebuilt online edition of Feynman's original physics lectures, with equations and figures that stay sharp at any zoom instead of a scanned PDF.

LibreTexts

A nonprofit library of free, editable college textbooks across 17 subjects, built so professors can remix chapters instead of assigning a $300 book.

🌀 Fun / Weird Internet

You Feel Like Shit

Walks you through a short sequence of low-effort yes/no questions about water, food, and sleep to figure out what's actually wrong when you can't tell yourself.

Cat Bounce

Click and the cats bounce. That's the entire site, and it's somehow still satisfying more than fifteen years in.

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33 sites this week. Found something broken or have a site to suggest? Reply to the email.