All Tools - Best free online tools for developers
ToolsViewer brings together some of the best free online tools for developers in one place. Filter by topic, browse a category, or search by name. No signup online tools, ever.
Built to Be the Best Free Online Tools for Developers
ToolsViewer was built around a clear goal: to offer some of the best free online tools for developers without requiring payment or registration. There is no account to create, no card to add, and no upgrade prompts interrupting your work. Open a tool, complete the task, and move on.
Every tool on this page runs client-side wherever possible, which means your HTML, your JSON, your regex patterns, and your email drafts stay on your device unless you choose to export or share them. That is the reasoning behind a no signup online tools approach - fewer steps between you and the result you need.
Tools on this page are organized into four categories: Viewers, Formatters, Converters, and Testers. Use the search box if you already know a tool's name, select a popular filter if you are searching by task, or browse each category to see what is available. New tools are added to this free online tools directory on an ongoing basis.
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- Free
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See It Before You Ship It2
Nothing is worse than sending an email or pushing a page live only to find a rendering bug five minutes later. The two tools in this group let you preview your work exactly as it will appear - in an inbox, in a browser, or on a phone screen - before anyone else sees it.
- Email ViewerPreview HTML and MJML emails with a free online email tester built for real QA work, not guesswork. Open .eml files directly, switch between Gmail and Outlook client views, flip on dark mode, and run a full email rendering test in seconds. If you've outgrown basic send-and-hope testing, this html email preview tool covers the checks that actually catch problems before your list does.Open Email Viewer
- HTML Viewer ProEdit and preview HTML, CSS, and JavaScript together with a free online html editor that updates as you type. Get a structure outline, run validate html online checks, compare two versions side by side with the diff tool, and export your finished markup to HTML, PDF, PNG, or JPG. It's the fastest way to go from a blank page to a shareable, browser-tested result.Open HTML Viewer Pro
Clean, Valid Code in Seconds2
Messy JSON and broken XML slow everyone down. These two formatters catch syntax errors instantly, pretty-print anything you paste, and turn cramped, unreadable data into something a human - or the next tool in your pipeline - can actually use.
- JSON Formatter & ConverterFormat, validate, and beautify JSON with a free online json formatter that flags errors as you type instead of after you've shipped them. Pretty-print for readability or minify for production, catch trailing commas and unquoted keys before they break a deploy, and turn Python dicts or JS objects into clean, valid JSON in a couple of clicks.Open JSON Formatter
- XML Formatter & ConverterFormat, validate, and beautify XML with a free online xml formatter built for SOAP, WSDL, and everyday markup alike. Pretty-print dense XML into something readable, minify it back down for production, validate structure against common errors, and run XPath queries without installing a desktop tool.Open XML Formatter
Convert Formats Without the Hassle2
Sometimes the format you have is not the format you need. These converters move data between JSON and XML in a single step, which removes the need to copy data into third-party sites that may store it on their own servers.
- JSON Formatter & ConverterConvert json to xml or CSV in a single click - a fast, free json to xml converter built for real conversion json xml jobs, not toy examples. Paste your data, pick a format, and download the result. No signup, no size limits that force you into a paid plan halfway through the job.Open JSON Formatter
- XML Formatter & ConverterConvert xml to json or CSV instantly with a converter built for anyone who needs to convert xml to json without installing desktop software. It handles nested elements, attributes, and namespaces cleanly, so the JSON you get out the other side is actually usable - not a jumbled mess you have to fix by hand.Open XML Formatter
Debug Smarter, Not Harder1
Regular expressions are powerful and famously easy to get wrong. This tool turns cryptic patterns into something you can actually see, test, and fix - without opening ten browser tabs just to look up syntax.
FAQs
Why does my JSON file show a syntax error when I try to use it?
JSON syntax errors are almost always caused by a trailing comma, an unquoted key, or a missing bracket. Open JSON Formatter & Converter, paste your file, and it highlights the exact line where the format breaks, so you can fix it in seconds instead of scanning the whole file by hand. This is especially useful when working with large configuration files or API responses, where a single misplaced character can be difficult to spot manually.
How can I preview an HTML email before sending it to a client or subscriber list?
Use Email Viewer to paste your HTML or MJML code and preview it as it would appear in Gmail, Outlook, Apple Mail, and Yahoo. This lets you catch layout breaks, broken images, or dark mode issues before the email reaches an inbox.
How do I convert an XML file into JSON without writing a script?
Open XML Formatter & Converter, paste or upload your XML, and use the built-in convert option to turn it into JSON instantly. This works well for API responses, configuration files, and SOAP or WSDL data that need to move into a JSON-based system.
Why is my regular expression not matching the text I expect?
A small mistake, such as an unescaped special character or the wrong quantifier, is the most common reason a pattern fails. RegEx Visualizer & Explainer highlights each match live as you type and steps through your pattern piece by piece, so you can see exactly where it stops matching.
How can I check if my HTML code will display correctly across different browsers?
HTML Viewer Pro renders your HTML, CSS, and JavaScript live as you edit, so you can preview layout, spacing, and structure before publishing. Use the validate feature to catch broken tags or unclosed elements that often cause rendering differences between browsers. This is particularly helpful when a page looks correct in one browser but breaks in another, since small markup errors are usually the underlying cause.
What is the easiest way to check an email for dark mode display issues?
Email Viewer includes a dark mode toggle that simulates how an HTML email will look when a recipient's email client switches to dark mode, which helps you catch invisible text or clashing background colours before sending a campaign.
How do I validate XML data before importing it into another system?
Paste your XML into XML Formatter & Converter and use the validate option to check for structural errors, unclosed tags, or invalid characters. This is useful before importing SOAP, WSDL, or configuration files into another application.
8.Can I convert JSON data into CSV or XML for a spreadsheet or another system?
Yes. JSON Formatter & Converter supports converting JSON into XML and CSV formats directly in your browser, which is useful when moving API data into a spreadsheet or a system that expects a different format.
How do I debug a regex pattern that only fails on certain inputs?
Use the step debugger inside RegEx Visualizer & Explainer to walk through your pattern against different sample inputs. It shows which part of the pattern matches and which part fails, which is faster than testing by trial and error in a code editor.
Are these tools free to use, and do I need to create an account?
Yes. Every tool on this page is part of a set of the best free online tools for developers, with no signup, no account, and no payment required to use the core features.