OsCommerce is a widely used open source shopping cart application that has a very active community of developers and contributors. There are hundreds of modifications, called contributions or add-ons that accomplish just about everything that you might need in an ecommerce storefront. OsCommerce is good for ecommerce start-ups because it is well supported through it's forums and can be set-up and run with minimal coding knowlege, and of course the low cost of an open source package.
Out of the box, OsCommerce is pretty stripped down. It's major drawbacks are search-engine unfriendliness and a presentation layer (user-interface) melded with the business-logic layer. Adding products can also be a real chore, especially if you have a large number of them. Here are the contributions I'd recommend adding to any OsCommerce installation:
Simple Template System (STS)
This powerful contribution separates presentation and business logic layers by creating a customizable templating system.
Because STS can generate a large number of database queries, especially in large stores, caching pages can greatly reduce server load.
Ultimate SEO URLS
Rewrites URLS (using apache mod_rewrite) based on product categories, product names and manufacturer names. Great for SEO!
Header Tags SEO V 3.0
Allows customization of page title, meta description, and meta keywords for product, category and manufacturer pages.
Easy Poplulate with Product Attributes
Allows upload and download of your products using a spreadsheet rather than the clumsy admin interface.
Allows you to define text content areas in your pages that can by edited using a tinymce interface in the admin area.