Useful guide on how improve e-mail marketing. It identifies three key elements of irresistible e-mail subject lines (“The Fundamentals”, “The Specifics” and “The Secret Sauce”) to help writing useful, unique and urgent subject lines http://www.copyblogger.com/email-subject-lines/
The periodic table of visualization methods
A tad bit weird at the first sight, but a really comprehensive overview of how to visualize things: The periodic table of visualization methods http://www.visual-literacy.org/periodic_table/periodic_table.html
The 5-minute conversion check scorecard
Brilliant: A Conversion Health-Check Scorecard http://unbounce.com/docs/conversion-scorecard.png
15 tools that reveal why people abandon your website
Helpful software for understanding your visitors and improving conversion: http://www.conversion-rate-experts.com/articles/understanding-your-visitors/