Top 10 Free Online Tools Every Developer Should Bookmark

Disclosure: I built these tools after repeatedly running into the same development tasks over and over again. They're free to use, browser-based, and process data locally whenever possible.

As developers, we spend a surprising amount of time doing small repetitive tasks:

Decoding JWTs
Comparing configuration files
Converting timestamps
Generating UUIDs
Building cron expressions
Inspecting API payloads

Most of these tasks shouldn't require installing software or opening an IDE.

Here are 10 browser-based tools that save me time almost every week.

Quick Overview

Tool	Common Use Case
File Size Calculator	Check upload limits
Gzip Base64 Decoder	Decode compressed API payloads
JWT Debugger	Inspect authentication tokens
Text Compare	Compare configs and code
JSON Escape / Unescape	Work with encoded JSON
Cron Expression Builder	Create and validate cron schedules
YAML Formatter	Validate YAML files
Epoch Converter	Convert timestamps
UUID Generator	Generate unique IDs
JSON to Go Struct	Generate Go models

1. File Size Calculator

The Problem

You need to check how large a file is before:

Uploading it to S3
Sending it as an email attachment
Passing it through an API with payload limits

The Solution

A file size calculator lets you drop a file and instantly view its size in:

Bytes
KB
MB
GB

No uploads required.

Tool:

https://www.solutiontoolkit.com/tools/file-size-calculator

This is surprisingly useful when debugging upload failures caused by hidden size limits.

2. Gzip Base64 Decoder and Encoder

The Problem

You receive an API response that looks like this:

H4sIAAAAAAAAA6tWKkktLlGyUlIqS...

Instead of JSON, you're staring at compressed gibberish.

The Solution

A Gzip + Base64 decoder can:

Decode Base64
Decompress Gzip payloads
Reveal the original JSON

Tool:

https://www.solutiontoolkit.com/tools/gzip-base64-encoder-decoder

I've used this frequently when debugging:

AWS Lambda responses
EventBridge events
Internal microservice communication

3. JWT Debugger

The Problem

Authentication suddenly stops working.

You have a JWT token, but no idea whether:

It's expired
It contains the expected claims
The wrong signing algorithm was used

Example

{
  "sub": "123",
  "role": "admin",
  "exp": 1788336000
}

A quick inspection immediately reveals whether the token is still valid.

The Solution

A JWT debugger decodes:

Header
Payload
Expiration time
Algorithm

Tool:

https://www.solutiontoolkit.com/tools/jwt-debugger

No libraries or command-line tools required.

4. Text Compare and Diff Tool

The Problem

You have two configuration files that should be identical but aren't behaving the same.

Common examples:

Kubernetes manifests
Terraform configs
SQL queries
YAML files
JSON payloads

The Solution

A diff tool highlights:

Additions
Deletions
Modifications

Tool:

https://www.solutiontoolkit.com/tools/text-comparison-tool

It's often faster than creating a temporary Git repository just to compare two snippets.

5. JSON Escape and Unescape Tool

The Problem

You encounter JSON that looks like this:

"{\"name\":\"John\",\"age\":30}"

Technically valid.

Practically unreadable.

The Solution

Convert between:

Escaped JSON:

"{\"name\":\"John\"}"

and readable JSON:

{
  "name": "John"
}

Tool:

https://www.solutiontoolkit.com/tools/json-escape-unescape

Useful when working with:

Lambda payloads
Elasticsearch queries
Nested API requests

6. Cron Expression Builder

The Problem

You need a schedule that runs:

Every weekday at 9 AM UTC

But cron syntax is easy to forget.

Is it:

0 9 * * 1-5

9 0 * * 1-5

The Solution

A cron builder translates schedules into plain English.

Example:

0 9 * * 1-5

becomes:

Every weekday at 09:00 UTC

Tool:

https://www.solutiontoolkit.com/tools/cron-expression-builder

Supports:

Standard cron
AWS EventBridge
Kubernetes CronJobs
Jenkins

7. YAML Formatter and Validator

The Problem

YAML is whitespace-sensitive.

One incorrect indentation level can break:

Kubernetes deployments
GitHub Actions workflows
Docker Compose files
Helm charts

The Solution

A YAML validator can:

Format YAML
Validate syntax
Highlight exact error locations

Tool:

https://www.solutiontoolkit.com/tools/yaml-formatter

Finding the exact line number saves a surprising amount of frustration.

8. Epoch and Timestamp Converter

The Problem

Your logs contain:

1748563200

You need to know:

What date and time is this?

The Solution

Convert:

Unix timestamps → Human-readable dates
Dates → Unix timestamps

Tool:

https://www.solutiontoolkit.com/tools/timestamp-converter

This is one of those tools every backend developer eventually bookmarks.

9. UUID Generator and Inspector

The Problem

You need:

Test identifiers
Seed data
Temporary resource IDs

Or you're working with UUID v7 and want to inspect its embedded timestamp.

The Solution

Generate and validate:

UUID v4
UUID v7

Tool:

https://www.solutiontoolkit.com/tools/uuid-generator

Being able to generate multiple UUIDs with one click is especially useful during testing.

10. JSON to Go Struct Converter

The Problem

You receive a JSON response like this:

{
  "user": {
    "id": 123,
    "name": "John"
  }
}

Now you need matching Go structs.

Writing them manually is repetitive and error-prone.

The Solution

Generate:

Struct definitions
Nested structs
Array types
JSON tags

Tool:

https://www.solutiontoolkit.com/tools/json-to-go-struct

What would normally take several minutes can often be done in seconds.

Honourable Mentions

A few more tools that developers may find useful:

YAML to JSON Converter
URL Encoder and Decoder
Base64 Encoder and Decoder
Word Frequency Counter
File Hash Generator

Available here:

https://www.solutiontoolkit.com/tools

Final Thoughts

The best developer tools aren't always the biggest frameworks or the most advanced IDE plugins.

Sometimes they're the small utilities that eliminate a repetitive task and save a few minutes every day.

Those minutes add up.

What browser-based developer tools do you find yourself using most often?

I'd love to discover a few new ones for my own bookmarks list.