PopTray is a free, opensource full-featured, customizable, e-mail notifier. Includes Spam management using rules and white/black lists.
Other Features include:

• Unlimited number of Accounts.
• Check mail from multiple accounts on a timer interval.
• Message Preview.
• Delete messages directly from the server.
• Plug-in Support: Protocol plug-ins (e.g. IMAP4, POP3SSL, Hotmail, etc.)
• Multi-lingual, translated into 30+ languages.
• OpenSource.