This website, www.actaddonshop.com, looks to be the largest selection of Act! add-ons anywhere.  They are created by a global community of certified developers. 

The Act! Marketplace has tools for different industries, categories, and platforms. Whether you're interested in mobility, quoting, project management, reporting, time-savers, or just general utilities, the Act! Marketplace has the products and the search & filter tools to help you find what you need to Act! better.

https://actaddonshop.com/