Online HTTP client

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/

http://web-sniffer.net/

Supports:

• list of user agents
• switch between HTTP/1.1 and HTTP/1.0
• test If-Modified-Since and If-Match headers
• support WWW-Authenticate
• search engine redirect added – all bots will be moved permanently to frontpage
• the URL may also be an Internationalized Domain Names (IDN)
• secure connections (HTTPS) allowed