Add-ons are one of the most important aspects of Dynamics NAV. A lot of the requirements for every industry are developed by third parties that service these particular industries.
When you search for "Dynamics NAV add-on" on your browser, you'll come across many sites that look like the official add-on site. Don't be fooled! These sites are nothing more than companies masking the domain to trick you into thinking it's a Microsoft site to sell you their own add-ons. Most of the add-ons listed are overpriced and unnecessary. So buyers beware!
When determining the add-ons, it's best to speak with your Dynamics NAV partner, because they should have your interest at heart for the implementation.
If you want to browse the add-ons available for Dynamics NAV, the official site is called Microsoft Pinpoint: http://dynamics-erp.pinpoint.microsoft.com/.