Exim for SMTP, Courier for POP3 and/or IMAP, and SquirrelMail or RoundCube for the web front-end - all available for Linux. It's good enough for cPanel, so it's

Exim for SMTP, Courier for POP3 and/or IMAP, and SquirrelMail or RoundCube for the web front-end - all available for Linux. It's good enough for cPanel, so it's good enough for me. Edit: Incidentally, it should be noted that all email is "web-based" these days...