Fairly recent [1] stats seem to point to:

  hotmail:  360m+ active accounts
  yahoo:    275m+ active accounts
  Gmail in 3rd, followed by AOL webmail [2}

Although yahoo! mail seems to have a more active user base as it's ahead on most site ranking systems [3].

Sources: [1] http://www.email-marketing-reports.com/metrics/email-statist... [2] http://techcrunch.com/2009/08/14/gmail-nudges-past-aol-email... [3] http://www.hitwise.com/us/datacenter/main/dashboard-10133.ht...