I am seeking a web based alternative to Microsoft Outlook Express.  A
completely online, and preferably free online email client that would
allow me to manage multiple POP3 and IMAP4 email accounts in a single
interface.

I don't know if this supports IMAP
our Cashette account includes a full-featured email account. 
  
  
 3000 MB of storage. No need to delete email you want to keep. 
  
  
 Consolidate your email accounts. You can access your multiple email
accounts at one place.
  
  
 Web-based. You can access your email anywhere you have access to the Internet. 
  
  
 POP3 enabled. Use mail software such as Outlook to access your email. 
  
  
 POP your Yahoo, Hotmail, AOL, and other email accounts. Just
consolidate these accounts at Cashette and then POP your Cashette
account
http://www.cashette.com/myCashette/CashetteMail/homepage.htm
http://www.emailaddresses.com/email_pop.htm Look around here for more.

This may http://www.cwazy.net/faq.html

Thanks for your comments.  Cashette appears to requre people to
'register' before you can receive an email from them.  cwazy.net is
not accepting new signups.  Any other ideas?

www.fastmail.fm supports IMAP I'm not sure if the free version supports POP.
www.emaildiscussions.com would be a good place to ask(it's free)

All email providers require registration,you would be forgiven for
using an assumed ID(at http://www.cashette.com/)or anywhere
else.Fastmail will require less info but they will still want some.

Thanks for your comments.  Again I want to manage multiple POP3 and
IMAP4 email accounts in a single web-based interface, similar to what
I can do with PC based Outlook Express.  www.fastmail.fm looks like a
good email service but it doesn't appear to allow you to manage
multiple accounts, like Outlook Express.  Concerning Cashette,
according to their website they require people who want to email you
to 'register' before you can receive their emails.  Thanks for the tip
on www.emaildiscussions.com, I will check that out.

It is possible to retrieve both POP and IMAP with fastmail
http://www.fastmail.fm/docs/faqparts/PopLinks.htm#NewPopServer
Imap server migration 
This will allow you to migrate all your emails from another IMAP
server to your account.

This is designed as a 'one-time' only action. It should not be used
regularly as you'll get a second copy of each email each time you run
it

Enter you server details, and then click 'Migrate' at the bottom of
the screen. Please note that large migrations can take up to an hour.
DO NOT run this migration a 2nd time, or you will get two copies of
everything! When the migration is complete, you will receive an "IMAP
Migration Report" email in your inbox.

While we have tested this feature with many different IMAP servers,
it's impossible to ensure that it works against all IMAP servers and
some bugs may still be lurking--if you have any problems please email
the details to webmaster@messagingengine.com. We hope you find this
service useful!