Anti-spam techniques

In the previous post, spammers techniques to get e-mail adresses were introduced. In this section, I will try to give an overview of how can spam be prevented, both by users and administrators.
The subject is broadly explained on Wikipedia.
However, I don't feel it would be enough just to put this one URL here, so below I'm linking to other sites related to this topic, that I found intresting. Looking into them should give you a broader perspective on how can spam controlled at different levels (government, big company, single user).
Here is an intresting article from O'Really Radar website, showing how spam can affect a single company.
Spamhaus organisation is fighting spam, particularly spam gangs. One of the interesting things that can be found on the webpage is Top 10 lists of spammers, countries and networks(with the greatest number of spam issues). You can read about how this organisation is trying to tackle spam on this webpage, additionaly as a support reading, I also recommend Wikipedia's article on DNSBL.
Spam Assasin is a very popular and useful tool both for users and admins of mail servers. The page holds many details about how the program works, which can give you a more detailed view on how the spam can be tackled.