If I have downloadable products, the standard installation of Zen-Cart will allow more than one of each download to be placed in the customer's cart ... but after they've checked out (and possibly paid for more than 1) the customer finds that they've only purchased a single download. This add-on limits the number of a each downloadable product to a single instance per cart.

The approach is simplistic and based on my current store where a product can be either physical or downloadable, as specified via a single option_name/option_value pair. However, the implementation is light-weight and requires no core-file overwrites.

You can download the add-on here.