Here are some useful ZenCart mods. Mods are kind of like plug-in software pieces that you can install to extend the functionality of ZenCart.

Prior to install mods, it is recommended that you look through the support threads to see if there are any incompatibilities issues that people might run across. For example, at the time of this writing, the Simple SEO URL mod will cause PayPal IPN to not work which is a serious defect. This was not mentioned in the install docs, but was seen in the support thread.

Quantity Discount Mod

Download here »
Allows discount in percentages or dollars if user purchase more than a certain quantity on a per item, per total items, or per category basis.

Group Pricing by Item

Download »
Different groups of users will see different price.

Backup MySQL Plugin

Download here »

Allows you to backup your MySQL database via the admin interface.

Admin Profiles

Download here »
Allows you to restrict access to certain parts of the admin interface for certain users.
Wiki and forum thread

Checkout Without Account

Wiki

Fast and Easy Checkout

Download here »
support thread here »
If you want to skip the checkout shipping page because your product is vitural, you can set that for individual products.