Firebug

Especially for Firefox

WATS

WATS is a SiteCatalyst HTTP monitor for Firefox. This add-on allows to visualize requests sent from websites to SiteCatalyst servers. It aims to make the testing process of the SiteCatalyst implementations easier. Key Features: * SiteCatalyst requests variables visualization. * SiteCatalyst requests history section. * SiteCatalyst requests history filter (filtered by reportSuiteID). * Descriptions can be set on SiteCatalyst variables. * Cookies set by the SiteCatalyst URL can be manipulated. * SiteCatalyst data can be exported into several formats. * Interface can be detached from a sidebar to an independent window.

Charles

HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP and SSL / HTTPS traffic between their machine and the Internet.

httpFox

Firefox Extension great for validating measure tags.

ObservePoint SiteAudit

Robust analytics tag auditing solution - crawls through every page of your site allowing you to find missing tags, broken links, and variables.

WASP

WASP allows you to test the state of your analytics tagging faster than ever without having to look at source code.

Google Chrome built-in debugger

Fiddler

Inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data.

Ghostery

Similar to Wasp (although not as comprehensive) Ghostery detects most cookies, trackers, pixels, beacons etc. on the page you are browsing. It also gives users the opportunity to 'block' any of the services detected.

DigitalPulse

DigitalPulse helps analytics professionals identify missing or incorrectly configured Omniture tags and broken links

HTTP Watch

integrated HTTP sniffer for IE and Firefox that provides new insights into how your website loads and performs.

Google Analytics Tracking Code Debugger for Chrome

This extension loads the debug version of the Google Analytics Javascript for the site you're browsing using Google Chrome. Messages include error messages + warnings which can tell you when your analytics tracking code is set up incorrectly. It also provides a detailed breakdown of each tracking beacon sent to GA.

IE Watch

debug HTTP and HTTPS traffic and easily analyze Cookies, POST and Query string information or locate HTML code by directly clicking in the web page.

Http DeBugger

Effectively view and debug http headers and http content.

Tamper Data

Use tamperdata to view and modify HTTP/HTTPS headers and post parameters.

GA?

Very simple Firefox add-on that verifies if Google Analytics is installed on the page you are browsing.