Email Marketing Statistics - Email marketing research

I think this is the most impressive publically-available collection of email marketing statistics I've come across. The contents table reads like a What's What:

Send, Open and Click-Throughs
    Most Popular Days to Send
    Email Send Times
    Open Times
    Sample Click-To-Open Rates
    Wednesday is "Opening Day"

Email Creative and Personalization
    Message Size, Number of Links, Subject Line Length
    Personalization

Email Delivery and ISPs
    Deliverability by ISP
    Which ISPs and Email Clients Block Images
    Sample Email List Composition by ISP

Email Privacy and Regulatory Compliance
    Percentage of E-Retailers Who Are Compliant With CAN-SPAM Opt-Out Provisions
    CAN-SPAM Compliance

Consumer Habits and Email Penetration
    Time of Day Online Users Check Personal Emails at Work
    When and Where Online Users Check Their Email
    Online Users Who Check Email While On Vacation
    Email Penetration Among Internet Users
    Total Number of Email Users in the U.S.
    Email Activity Among Internet Users

Email Marketing Industry Growth and Trends
    Trend of Email Volume in the U.S.
    Compounded Annual Growth Rate for Email
    Email Marketing vs TV Advertising
    Email Outpulls Catalogs for Direct Marketer Web Sites

Retail Email Marketing
    Percentage of E-Retailers Who Are Compliant With CAN-SPAM Opt-Out Provisions
    Email Click-Through Rates For E-Retailers
    Email Conversion Rates For E-Retailers
    E-Retailers' Percent of Online Sales Attributed to Email Marketing
    Reasons E-Retailers Are Growing Their Email Marketing
    Email Activity by E-Retailers Now Versus A Year Ago
    Subject Lines That Will Motivate Moms to Open a Retail Email Message
    How Often Moms Would Like to Be Notified By Retail Emails
    Product Information in Retail Emails That Will Motivate Moms to Click
    Eretailer Email Frequency
... phew

Link: Email Marketing Statistics - Email marketing research.