Mturk Suite is currently the most widely used Mechanical Turk script. It offers a variety of useful features including a customizable HIT scraper, a Panda manager, and a completed HIT tracker, as well as several quality of life changes to the Mechanical Turk site. This is the first script I would recommend most people download and learn how to use. This alone can significantly increase your turking proficiency.
Panda Crazy is a feature rich Panda manager that allows for further Panda customization than other available scripts offer, including the one offered by Mturk Suite. Mastering the art of Pandas can really help you catch the best HITs Mturk has to offer, and this script can help.
Turkerview is a review platform which allows users to leave reviews for both HITs and requesters. Installing this script will allow review information to be displayed direcectly on Mechanical Turk. This can really help you determine which HITs are worth doing and which ones to avoid.
Mturk engine offers a suite of tools to help you find, catch, and manage HITs. Among these tool are a HIT scraper, Panda manager, HIT tracker, and a queue manager. All features are contained in a clean, easily navigatable user interface.
The original Mechanical Turk HIT review service. Turkopticon allows users to review HITs or requesters and will display review information directly on Mturk. While Turkopticon as fallen out of use lately due to the rise of Turkerview, it is still a valuable resource (although I would only install one or the other.)