As a backend developer, I spend a surprising amount of time on small but repetitive tasks — converting a subnet mask, decoding a JWT, formatting a SQL query, or generating test data before a demo. These aren't hard problems, but they slow you down when you have to hunt for the right tool every time.
Here are 10 free, browser-based tools I keep coming back to. No login, no install — just open and use.
1. IP / Subnet Calculator
Handy Dev Tools — IP Calculator
Paste a CIDR like 192.168.1.0/24 and instantly get the network address, broadcast address, wildcard mask, usable host range, and binary/hex notation. Pair it with the Subnet Split tool to divide a network into equal subnets by count, host requirement, or prefix length — results exportable as CSV.
Useful when designing VPC address spaces or writing firewall rules and you need to double-check your math fast.
2. CloudFront Signed URL Generator
Handy Dev Tools — CloudFront Signer
Generates AWS CloudFront signed URLs entirely in the browser using RSA-SHA1. Supports both Canned Policy (expiration only) and Custom Policy (IP restriction, start time, wildcard resource patterns). Your private key never leaves the browser — it's handled in memory only.
I haven't found another free tool that handles Custom Policy with IP/CIDR restrictions. Handy for quickly testing signed URL behavior before wiring it into your application code.
3. JWT Decoder
The classic. Paste a JWT and immediately see the decoded header, payload, and signature verification status. jwt.io has been around forever and works exactly as expected — no reason to switch if you're already using it.
4. S3 Path Formatter
Handy Dev Tools — S3 Path Formatter
Converts between every S3 format in one place: s3:// URI, ARN, path-style HTTP URL, virtual-hosted URL, CloudFront URL, and AWS Console URL. Paste any format, get all the others instantly.
Saves the mental overhead of remembering which format the AWS CLI, SDK, IAM policy, or CloudFront origin config expects.
5. JSON / YAML / SQL Formatter
Fast, in-browser formatters for JSON, YAML, SQL (with dialect support), XML, GraphQL, and Markdown. Nothing surprising here, but having them all under one URL means fewer tabs.
For SQL specifically, the formatter preserves semantics while normalizing indentation and keyword casing — useful before committing a migration file.
6. Regex Tester
Still the best dedicated regex tool available. Supports PCRE, JavaScript, Python, Go, and Java with detailed match explanations and a built-in library of saved expressions. If you're working with complex patterns, regex101 is worth the dedicated tab.
7. Test Data Generator
Handy Dev Tools — Test Data Generator
Generate up to 10,000 rows of realistic dummy data across 50+ field types: UUIDs, names, addresses, phone numbers, IP addresses, JWT tokens, API keys, credit card numbers, Japanese era dates, and more. Output formats include JSON, CSV, SQL INSERT, and Laravel Seeder.
The Laravel Seeder output is the feature I use most — paste it directly into a DatabaseSeeder.php and you're done.
8. Hash Generator
Handy Dev Tools — Hash Generator
Supports MD5, SHA-1, SHA-256/512, SHA-3, bcrypt, scrypt, Argon2, HMAC variants, and CRC32 — for both text and file input. Useful for verifying download checksums or quickly checking what a bcrypt hash looks like for a given cost factor.
All hashing runs client-side, which matters when you're working with anything sensitive.
9. cURL to Code Converter
Paste a cURL command, get equivalent code in Python (requests), JavaScript (fetch/axios), PHP, Go, Ruby, and more. Invaluable when you're copying a request from browser DevTools and need to reproduce it in your backend code.
10. HTTP Status Code Search
Handy Dev Tools — HTTP Status Search
Search by code number or keyword ("redirect", "auth", "rate") and get the status name, meaning, common causes, and when to use it. Works offline — the database is bundled in the page.
Quick reference when you're deciding between 400 vs 422, or 301 vs 307.
Wrapping Up
Most of these tools do one thing well and stay out of your way. The ones I reach for most often are the subnet calculator, S3 path formatter, and test data generator — they solve problems that are just tedious enough to slow down a workflow but not complex enough to warrant writing custom scripts.
If you know a tool that belongs on this list, drop it in the comments.
Tools 1, 2, 4, 5, 7, 8, 10 are from Handy Dev Tools — a free, open toolkit I've been building for backend developers. 69+ tools total, all browser-based, available in 7 languages.
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