Think you can write a better web mail interface?  I know I do .  I'll have to take a look at it a bit more when I get a chance.

If you're interested, here are a few good starting points for ya:

• A small introduction for a C# client implementation.  Looks like a good starting point to get your code going.
• RFC1939 and subsequent extension, RFC2449.   The official specification for the POP3 protocol; a good reference to clear up any ambiguities when writing your implementation.
• RFC2821.  The official specification for SMTP (used for sending email messages).  I dealt quite heavily with this RFC when I was at Factiva.  Quite an interesting spec, but probably better to use CDOSYS instead if you have that available and you don't need full control over the SMTP implementation.
• An article by Duncan Mackenzie on writing a custom client.  Duncan also has a neat little ballon popup implemented.
  
• A .Net 2.0 library for POP3 communication.  Looks full featured and documented...good starting point if you're not interested in the intricacies of POP3.
  
• A good sample POP3 session "conversation" example.  It helps to understand how the messages are typically used in a session.

Will be interesting, I'm looking forward to getting some free time to work on this.

I'm debating whether it makes sense, from a performance perspective, to use a sort of POP3-SQL Server synchronizer to move messages from one or multiple accounts into one repository for fast indexing and lookup.