Zotero is open-source, and it is easy for anyone to create additional tools to add functionality to the program. Over the years, many add-ons have been created that give users the ability to customise Zotero to their needs and preferences.
To install these add-ons, the usual procedure is to download the file (with an xpi. extension) to your computer, then go to the Tools -> Add-ons menu, click on the gear on the right, select Install Add-on from File and select the file.
For a complete list of Zotero add-ons, click here.
ZoteroPreview adds a Preview tab with a citation preview for the item.
Another way to preview a citation in all the styles you have in your Zotero is to use the Style Preview pane, a built-in feature of Zotero. Preferences menu -> Cite, and click on the Style Preview button. Select an item and click on Refresh.
Zutilo is an extension that adds several features to Zotero, such as:
Go to the menu Tools -> Zutilo Preferences to select the features you want. Each time you will right-click on a reference, you will have an additional Zutilo menu with these functions.