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.
Zotero Tag is a plugin for managing tags, with batch processing and the ability to set custom rules.
Zotero Storage Scanner allows you to clean up attachment files, by locating the missing or duplicate files.
This plugin can auto-fetch DOIs for new items, shorten DOIs using http://shortdoi.org, and check the validity of DOIs.
Links to web resources often break. The Zotero Robust Links extension allows users to create their own robust links from within Zotero.
This plugin makes it easier to merge duplicates (and merge them in bulk).
This plugin allows you to analyse your Zotero library of references with R to create graphics and csv files. A more user-friendly variant is available.