Free online developer tools

W3C validator that checks HTML documents for errors.

Fill a grid and set column alignment to get a Markdown table, with offline use and no uploads.

Searchable reference of AWS IAM actions, resources and condition keys for each service.

Generates AWS IAM, S3, SNS and SQS policy JSON from form selections.

Spins up a mock REST, SOAP or GraphQL endpoint with rules and generated responses, and logs incoming requests.

Web editor for writing code on a CircuitPython board over USB or WiFi.

Builds a crontab line from schedule fields, command and output options.

Lists which CSS properties trigger layout, paint or composite work in browser engines.

Helps you pick the right git command by describing the change you want and the repository state.

Compares cloud instance types for a Kubernetes cluster by how much capacity is left for pods.

Fills in Deployment, Service and ConfigMap manifests from a form to produce Kubernetes YAML.

Public inboxes for catching test emails and verification codes during QA.

Assembles or disassembles code for several CPU architectures using Keystone and Capstone.

Converts SQL between database dialects such as Oracle, SQL Server, MySQL and PostgreSQL.

Runs SQL against several database engines online and shares the fiddle by link.

Counts tokens for GPT, Claude, Gemini and Grok text and shows the token split and costly words.

Shows which browsers and versions a Browserslist query string would target.

Converts between CBOR binary in hex and the diagnostic text notation.

Translates C and C++ declarations such as function pointers into plain English and back.

Visualises a CIDR block or IP range with its subnets and masks.

Translates source code between more than 60 programming languages; the page says free tools work without signup.

Collection of converters and formatters for JSON, YAML, XML and similar formats.

Generates cron expressions from a visual form.

Edits cron schedule expressions and explains them in plain English.