Here are Salesforce Addons & Tools that I use. If you use others, please share in the comments.
Declarative Lookup Rollup Summary – This is an open source Salesforce managed package that lets one declaritively define a rollup-summary on a lookup or master-detail. It aggregates a value from child records onto a parent record using the configuration defined. This was originally developed by Andy Fawcett and now has many contributors.
Metadata API Apex Wrapper – This Salesforce managed package lets one call the Salesforce Metadata API from Salesforce using Apex. By allowing creating, updating, or deleting metadata programmatically, this opens up more declarative possibilities from Salesforce natively. For example, creating triggers and apex classes from Visualforce and Lightning pages or automating the install or uninstall of packages.
Workbench – A website hosted by Salesforce that has various utilities for admins and developers. Examples include a SOQL query builder, REST explorer, Schema browser, and bulk data exporter and importer.
Salesforce Boostr – Boostr Chrome extension for Salesforce.com. Started by mattsimonis in February, 2016, this extension provides various useful enhancements to the Salesforce UI when logged in. Examples include clearing out the text in the Admin Search’s textbox and showing the number of fields selected on the field history tracking page. Salesforce Boostr Chrome Extension Link