I was trying out a new tool called URL Trends this morning - you should give it a try, it's a really great tool which tracks how a domain does over time. Each month it tracks Alexa ranking, Yahoo! backlinks, MSN backlinks, & Google backlinks.
Anyway, when you pull a trend report for a domain name it also gives you an overview of all related sites. In my case there were two results only - of which one was completely irrelevant - but the other one is actually a very interesting website called Direct Email Marketing Guide. This site aims to teach you how to get started with an email marketing campaign.
If you look beyond the ads, there's some really good information on this website. Check it out.