Usually you're consuming web services providing JSON (JavaScript Object Notation) when you're doing mashups. To illustrate why the add-on is useful, we will use a geocoder service, such as Nominatim, to retrieve position for a postal code. A geocoder is a tool to match addresses with geographic coordinates.

For example, typing http://nominatim.openstreetmap.org/search?q=35+Livery+Street+Birmingham&format=json&limit=1 in Chrome will send you the following screenshot:

With the extension, you will directly get an indented content, collapsible.

So what is the difference? When it's required to correctly reuse data, we must understand received data and with this plugin, the return content is human readable! Furthermore, OpenLayers can consume GeoJSON, a subset JSON notation dedicated to maps data.

Download it at https://chrome.google.com/webstore/detail/jsonview/chklaanhfefbnpoihckbnefhakgolnmc.

When you are making a custom UI, you always have to try different colors.

You can also retrieve a color you like from another website or an image. In this situation, you will not want to always go back and forth between a graphic tool such as Gimp or Photoshop and your browser. So in this case, you can install ColorZilla. How is this related to OpenLayers? When you want to make customize controls elements, this is an invaluable tool. Go to http://www.colorzilla.com/chrome/ to retrieve it. It's also a plugin available for Firefox.