This typically includes techniques such as IP Blocks, White-lists and Black-Lists and Content Scanners.  The spam is intended to insert links into your site. Passive protection focuses on using content scanners which validate the content, the source & method of delivery against extensive databases of bad links, emails, content, blacklisted IPs & domains to stop the spammer from getting his input in.

These checking databases are fed input from a huge number of sites and users that subscribe to using the databases and they in turn report malicious content.  The important thing about these databases are that they are updated continuously and are true to the spirit of open source being contributed to on a everyday basis making them a comprehensive source of information.

There are many examples of such databases and include Akismet, Mollom, Project Honey Pot, Bad Behavior to came but a few.

This 'Captcha less' Spam protection can be very effective mainly since in most cases it helps the system become even more stronger.  One downside is that on a busy site it can add some considerable overhead to the processing load, and possibly slow the user's site experience.