Where does Spam come from?

I've wondered for a while why I get Spam. There are a few hypotheses:

• I'm the admin of some websites that receive thousands of hits per month
  
• One of my addresses is comprised of my first initial and my surname, both of which are very common
• In my time on the net I've signed up to many email lists, web sites and forums. I've also had to give my email address to download software and do other internet activities

There are of course many other ways you can get Spam, and it is probably more that I have done all of these things rather than just one.

I'm going to do an experiment to test out the three spam-getting methods above. I'll get a domain, run a mail server on it and create some email accounts. Some email addresses will be cryptic ones that you'd never guess like akjalfdslkjasdlkjf324@mydomain.com, while others will be constructed from a list of baby names. I'll leave the accounts open and receiving mail for a few months and report back on my results.