Product's meta description is a great SEO tool.

This extension generates meta description for products which don't have meta descriptions from relevant words in product's title and model and category's title.

Supports OpenCart Version 1.5.x (vqmod required)

Multi-language support. No files will be overridden. Easy VQMOD install.

This extension is included in Opencart SEO PACK PRO. It contains 20+ extensions and you can save more than $150 by purchasing the full Opencart SEO Pack PRO:

http://www.opencart.com/index.php?route=extension/extension/info&extension_id=6182

UPDATE:
Meta Description Generator v 2.0 has been released:
- You can parameterize your generated descriptions
- 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.