impromptu presentation on the best gadgets in Wikidata, based on personal experience:

MediaWiki namespace

enable these through Preferences > Gadgets

• Merge: makes it possible to merge duplicate items. Adds menu item in top right corner under "More"
• Move: move a site link from one item to another
• KeyShortcuts: prevent RSI while editing individual items by enabling this and hitting the A-key to add a statement, L-key for editing the label and D-key for description. Also consider expanded edition by Nikki
• Reasonator: adds a link in the left side-bar that brings you to a visually more attractive overview of that item and those that are linked to it.
• DuplicateReferences: makes it possible to copy a reference from one statement to another.
• currentDate: automatically add current date to the retrieved (P813) property.

Enhance user interface

the following gadgets are still being maintained by individual users and are a bit more experimental. Enable them by editing your common.js or common.css

• identifierInput.js: strips the identifier from a full URL when added as a property or reference
• Compact items: stylesheet that makes the Wikidata interface more compact
• RearrangeValues: makes it possible to rearrange the multiple values of a property of an item

Edit items

• Move claims: makes it possible to move/copy a claim to a different item or a different property within the item