Due to the sheer volume of extensions available, it is impossible to list them all. Instead, here is a list of some of the most recognized Magento extension developers and a selection of their best extensions:
Aheadworks (goo.gl/FSYT9Y): Take a look at Follow Up Email for remarketing and AJAX Cart Pro for CRO and usability
Fooman (goo.gl/JZ3hsb): Take a look at Fooman Speedster for Magento load speed optimization and Fooman Google Analytics+ for enhanced Google Analytics tracking
Mage Store (goo.gl/JxwCJK): Mainly known for their Affiliate+ extension
M2E (goo.gl/KwtIwR): Used for integration into eBay stores and Amazon
Amasty (goo.gl/gX3BUQ): Used for a wide range of usability improvements, such as improved layered navigation
Unlike this book, this list could quite literally never end as more and more companies develop increasingly popular Magento extensions.