Related Products are perfect to keep customer on your site as well as a good SEO tool (more relevant internal links).

Related Product Generator, based on a complex algorithm, is a powerful tool which is very useful if you have many products and it takes plenty of time to find related products for them. In a couple of seconds, Related Product Generator will generate up to 5 related product for each product.

This extension is included in Opencart SEO PACK. It contains 11+ extensions and you can save more than $40 by purchasing the full Opencart SEO PACK:

http://www.opencart.com/index.php?route=extension/extension/info&extension_id=4653

UPDATE:
Related Products Generator v 2.0 has been released:
- You can set the maximum number of related products
- Clear SEO is included for FREE
- SEO report is included for FREE

DEMO:
http://ocdemos.womgoo.com/admin
user:demo
pass:demo

This extension can be found in admin area Catalog -> SEO Pack.

*** This extension is not compatible with OC 2.x. The version for OC 2.x and OC 3.x is included in Opencart SEO Pack PRO which also includes all the other SEO related extensions and features.