The product bundles module allows your users to add certain products as optional accessories for other products. 

Bundled products can be hidden from view, in order not to display on the search results pages on their own, in which case they will only display on the listing details page of the parent product.

As an example, a seller could create a product called "Computer", and as optional bundled products s/he could add "Printer", "External HDD" etc.

Bundled products can be grouped for single selection, for example if for the "Computer" the seller would offer RAM upgrades, s/he will be able to add several RAM modules but only allow one of them to be included with the order.