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Of course, "essential" is a matter of opinion, but here are several I use a lot (besides apps pre-installed on the phone, such as Maps):

• Wikipedia (Official Wikipedia App from Wikimedia) - generally quicker to use than accessing the Wikipedia Website through the browser.
• ColorNote - Lets you keep little notes and to-do lists, with alarms. Includes a widget to "stick" a note on your Home screen.
• ToMarket Pro - Fine shopping list app for going to the grocery store. Good customization in adding items to the database.
• QuickOffice Pro - Paid version of the office-document app that came pre-installed. Great for reading PDFs, creating short documents, etc.