It's easy to extend Spree by writing your own extension. There are already over one hundred third party extensions listed in our official registry.
Spree Zero Stock Products Restore show_zero_stock_products functionality in Spree 2.0+
Product Line Adds ability to display related products on a product page.
Turbo turbo project
SPREE NOVALNET PAYMENT GATEWAY Spree Extension for payments via Novalnet Payment Gateway
Dwolla Official Dwolla for Spree
Infinite Scroll Infinite scroll for product listing
spree_lookbook Spree Look Book for creating a look from a group of different products.
SlidePay Spree SlidePay plugin for SpreeCommerce
Spree Admin Bar Adds an admin bar to the shop front-end.