The Essential Developer Toolkit: 10 Must-Have Online Tools

# The Essential Developer Toolkit: 10 Must-Have Online Tools

I keep a folder of bookmarked browser tools for the stuff that isn't worth opening an IDE for — formatting a JSON response, checking a regex, decoding a JWT. None of these need installation, none of them see my data, and most of them I reach for several times a day.

Here are the ten I actually use, what I use them for, and where to find them.

JSON Formatter

The one I open most. Paste a minified API response, get readable indented JSON with syntax errors flagged by line number. I use it whenever a third-party endpoint returns something I can't visually parse — which, with minified production APIs, is most of the time.

Try JSON Formatter →

Regex Tester

I've stopped pretending I can write regex correctly on the first try. I paste the pattern, paste the test string, and see matches highlighted in real time. It saves me from the "works on my input, breaks in production" problem.

Try Regex Tester →

Diff Checker

For comparing two versions of a config file, a YAML manifest, or a chunk of someone's PR. Side-by-side highlighting makes it obvious what actually changed, which is faster than reading both top to bottom.

Try Diff Checker →

Cron Parser

I can never remember whether the day-of-week field comes before or after the month. Paste the expression, see the next few run times in plain English. Useful before deploying a schedule that would otherwise fire at 3 AM on Sunday.

Try Cron Parser →

Base64 Encoder

Embedding a small logo or icon directly into CSS to skip an extra request, decoding a data URI someone handed me, or inspecting what's actually inside a JWT's payload. Handles both directions.

Try Base64 Encoder →

Hash Generator

Verifying a downloaded file against a published checksum, or generating one for a file I'm distributing. MD5, SHA-1, SHA-256 — pick the algorithm, paste the input, copy the digest.

Try Hash Generator →

JWT Decoder

When auth breaks and I need to see what's actually in the token — the exp claim, the roles, the issuer. Paste the JWT, read the decoded header and payload. No secret needed for inspection; signature verification is there when you need it.

Try JWT Decoder →

Markdown Previewer

Writing a README or a docs page and wanting to see the rendered result before pushing. Live preview while I type, so I catch broken links and bad formatting before they hit the repo.

Try Markdown Previewer →

Number Base Converter

Converting between decimal, hex, binary, and octal. Mostly for color codes (hex ↔ rgb), the occasional bitwise flag, and reading memory addresses in stack traces.

Try Number Base Converter →

Slug Generator

Turning post titles into URL-safe slugs — stripping accents, handling spaces, picking a separator. Beats writing the normalization logic myself every time.

Try Slug Generator →

How I Actually Use These

The trick isn't memorizing what each does — it's having them one click away. I keep them in a bookmark folder ordered by how often I reach for them, so the JSON Formatter sits first and the Slug Generator last. When something comes up, I don't think about which tool to open; I just open it.

If you only bookmark one, make it the JSON Formatter. If you bookmark three, add Regex Tester and Diff Checker. The rest earn their spot once you hit them often enough to notice.

The full set lives at FreeToolJet — all free, all in-browser, none of them send your input anywhere.