Ok there are a lot of people interested in the osCommerce open-source (free for non techies!) e-commerce package.
So I'm starting this thread so people can post their favourite customisations for osCommerce - contributions or otherwise.

For those new to osCommerce, please visit www.oscommerce.com

It would be helpful if everyone could set out their replies in the same way:

1. Customisation Name
2. Customisation Purpose
3. Customisation URL
4. Any other comments

See sample below...

Name: Basic Template Structure v1.3

Purpose: Customising oscommerce is an absolute nightmare - especially for beginners. This contribution puts switches the site onto a template system from which users can easily switch between templates and edit the look and feel of the site from the one (template) folder. If also give the user much easier control over the cascading stylesheets support for the site.

URL: http://www.oscommerce.com/community/contributions,1263

Other: Highly recommended, this has been a godsend on a number of projects - thanks to the authors.
Instruction are included in the download, and you may have to persevere with the changes (especially if it is not a fresh install), but it is well worth it.

Name: Actual Attributes Price v1.2.1

Purpose: Ok this is blatant self-promotion for something I have contributed to, but it's an extremely useful contribution. By default product attribute options (ie colour, weight, size) only show their affect on the product's price, rather than what the new price is (ie +£10 rather than say £35). This contribution displays the product's exact price with whatever attribute is selected.

URL: http://www.oscommerce.com/community/contributions,1716

Other: Much more user friendly way of doing things