To complete the list :
Captcha free
- OnlineCurl : http://onlinecurl.com/
- GoogleBot Simulator : http://www.dnsqueries.com/en/googlebot_simulator.php
- getaCert HTML debugger : http://www.getacert.com/cert/websniffer.py
- TestUri : http://testuri.org/
- REST client - HTTP request: https://toolki.com/en/http-request/
- AskApache::Raw HTTP Header Debugger : http://www.askapache.com/online-tools/http-headers-tool/
With Captcha
- REDbot : http://redbot.org/
- Rex Swain's HTTP Viewer : http://www.rexswain.com/httpview.html
- Web Sniffer : http://web-sniffer.net/
Dormant
- Hurl : http://hurl.it/
- Hurl.eu : http://hurl.eu
- Web-sniffer Me : http://www.web-sniffer.me/
Misc.
Tools not primarily intented to be online HTTP client but you can use them for :
- The Right API http://www.therightapi.com/test (support OAuth 2.0 and some data types such as HTML, JSON, XML, Text...)
- Online Tidy up: http://infohound.net/tidy/
- Try Jsoup: http://try.jsoup.org (Click the Fetch URL button to enter your url)
- HTML Beautifier: https://dotmaui.com/htmlbeautify/
