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Google Tag Assistant (Essential) https://chrome.google.com/webstore/detail/tag-assistant-by-google/kejbdjndbnbjgmefkgdddjlbokphdefk
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Google Analytics Debugger (Recommended) https://chrome.google.com/webstore/detail/google-analytics-debugger/jnkmfdileelhofjcijamephohjechhna
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WASP (Recommended) https://chrome.google.com/webstore/detail/wasp-web-analytics-soluti/niaoghengfohplclhbjnjheodgkejpih
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ObservePoint Tag Debugger https://chrome.google.com/webstore/detail/observepoint-tag-debugger/daejfbkjipkgidckemjjafiomfeabemo
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Ghostery https://chrome.google.com/webstore/detail/ghostery/mlomiejdfkolichcflejclcbmpeaniij
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Adobe/Omniture Analytics Debugger https://chrome.google.com/webstore/detail/analytics-debugger/olkcipmnccchnnikelenphfdeigajlcl/details
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Dataslayer https://chrome.google.com/webstore/detail/dataslayer/ikbablmmjldhamhcldjjigniffkkjgpo
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DataLayer Inspector (new) https://chrome.google.com/webstore/detail/datalayer-inspector/kmcbdogdandhihllalknlcjfpdjcleom
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Tampermonkey Injector for Testing on Live https://chrome.google.com/webstore/detail/tampermonkey/dhdgffkkebhmkfjojejmpbldmpobfkfo https://gist.github.com/tfayyaz/19aa32ae4d5a2dc98a5b
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GTM Container Injector for Testing on Live http://www.conversionworks.co.uk/GTMInjector/Beans%20and%20noses%20-%20public%20edition.pdf http://www.conversionworks.co.uk/GTMInjector/GTM%20Extension%203.zip
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Code Formatter for GTM https://chrome.google.com/webstore/detail/code-editor-for-gtm/nolgbicpmekdflgalbbljkplmgabjmdm
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JSON Formatter https://chrome.google.com/webstore/detail/json-formatter/bcjindcccaagfpapjjmafapmmgkkhgoa
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Live HTTP Headers https://chrome.google.com/webstore/detail/live-http-headers/iaiioopjkcekapmldfgbebdclcnpgnlo
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Fiddler Headers https://chrome.google.com/webstore/detail/fiddler/hkknfnifmbannmgkdliadghepbneplka?hl=en
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Firebug Lite https://chrome.google.com/webstore/detail/firebug-lite-for-google-c/bmagokdooijbeehmkpknfglimnifench
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Proxy SwitchySharp https://chrome.google.com/webstore/detail/proxy-switchysharp/dpplabbmogkhghncfbfdeeokoefdjegm
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Web Developer Toolbar https://chrome.google.com/webstore/detail/web-developer/bfbameneiokkgbdmiekhjnmfkcnldhhm
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Clear Cache & Cookies Button https://chrome.google.com/webstore/detail/clear-cache/cppjkneekbjaeellbfkmgnhonkkjfpdn
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User-Agent Switcher https://chrome.google.com/webstore/detail/user-agent-switcher/lkmofgnohbedopheiphabfhfjgkhfcgf
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GA settings Copy and Paste https://chrome.google.com/webstore/detail/ga-copy-and-paste/imgngfbfofnhpgdhoejahgpggoclgcom
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LastPass: Free Password Manager https://chrome.google.com/webstore/detail/lastpass-free-password-ma/hdokiejnpimakedhajhdlcegeplioahd
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SiteCatalyst Debugger https://chrome.google.com/webstore/detail/sitecatalyst-debugger/bdingoflfadhnjohjaplginnpjeclmof
More listed here in the App Gallery: http://www.google.com/analytics/apps/results?category=Widgets%20%26%20Gadgets
G+ url: https://plus.google.com/117298997433687198127/posts/ChPMqr6XQRg