Every developer has a list of bookmarked tools they reach for constantly. Not the fancy ones they tweet about — the boring, reliable ones they quietly use 10 times a week.
Here is mine. All free, no signup required for core features, and fast enough to not interrupt your flow.
1. CalcProTool — 263 calculators, one tab
calcprotool.com
I built this one, so I am biased — but I also use it more than any other tool on this list.
The use cases keep surprising me. Last week I needed a binary-to-hex converter for a debugging session, a mortgage amortization breakdown for a client project, and a quick calorie estimate before deciding whether lunch was worth it. All three were faster to find at CalcProTool than searching Google and landing on an ad-heavy result.
What I actually find useful as a developer:
- Binary / Hex calculator — converts in both directions, handles arithmetic
- Percentage calculator — three modes: find X% of Y, what % is X of Y, percentage change
- Age calculator — exact days/weeks/years, useful for date-diff sanity checks
- Data storage converter — bytes, KB, MB, GB, TB with no rounding weirdness
No ads, no login, all calculations run client-side in the browser. Source is Next.js 15 with TypeScript.
2. Ray.so — beautiful code screenshots
ray.so
When you need to share a code snippet in a PR comment, Slack message, or blog post and do not want to paste raw text. Supports syntax highlighting for most languages, multiple themes, and exports as PNG or SVG.
Takes 20 seconds. Looks significantly better than a screenshot of your terminal.
3. Squoosh — image compression in the browser
squoosh.app
Google's image compression tool. Drag in a PNG or JPEG, choose your output format (WebP, AVIF, MozJPEG), adjust quality, and download. The before/after visual preview makes it easy to find the right quality-to-size tradeoff.
I use this before deploying any project with images. A 400KB PNG often compresses to 40KB WebP with no visible quality loss. That is a meaningful LCP improvement for zero development effort.
4. regex101.com — regex debugger with explanation
regex101.com
Everyone bookmarks this one. Write your regex, paste test strings, and see exactly which parts match and why. The "Explanation" panel on the right breaks down every token in plain English.
The "Quiz" mode is also a genuinely good way to get better at regex. Less embarrassing than Googling "how to match optional whitespace" for the fifteenth time.
5. transform.tools — data format converter
transform.tools
Converts between data formats: JSON to TypeScript interfaces, JSON to GraphQL, CSS to JS object, SVG to JSX, and about 40 other transformations.
The one I use most is JSON → TypeScript interfaces. Paste in an API response, get a typed interface back in two seconds. Not always perfect, but 80% of the manual work done instantly.
6. OverAPI.com — cheat sheets for everything
overapi.com
A massive collection of cheat sheets for languages and frameworks. Each cheat sheet is a visual list of methods/functions that links to the official documentation. Not for learning — for that moment when you know a method exists but cannot remember the exact name or signature.
Covers JavaScript, Python, PHP, Ruby, CSS, Linux, Git, and more.
7. Wappalyzer — tech stack detector
wappalyzer.com (browser extension)
Visit any website and immediately see what it is built with: framework, CMS, analytics tools, CDN, payment processors. Useful when a client asks "can we build something like X?" — you can check in 2 seconds whether X is on WordPress or a custom React app.
Also genuinely interesting for competitive research. Knowing a competitor's tech stack tells you a lot about their engineering capacity and deployment constraints.
The pattern I notice
All seven of these tools share the same qualities:
- No forced signup for core functionality
- Instant results — no loading spinners, no "processing your request"
- Single clear purpose — they do one thing well
- Work offline or near-offline — most run calculations client-side
The tools that frustrate me most are the ones that make you create an account to use a basic feature, or show you a loading screen for something that could run in 10ms in a browser tab.
If you are building developer tools or utility sites, that is the bar worth aiming for.
What free tools do you use daily? Drop them in the comments — always looking to expand the list.
I built CalcProTool — a free online calculator with 263 tools for finance, health, math, and more. No signup, no ads, all calculations run in your browser.
