Alerts
GrowlMail is a favourite. Simple and efficient — it takes me to Mail (does not try to replicate the features of Mail).
Filing
MsgFiler is a favourite. Fast, functional and easy to remember — just one accelerator, -9
Viewing
Letterbox is a favourite. View messages to the right of (not below) the list of messages.
Multi-purpose
Herald and Mail.appetizer are similar to each other.
Mail.appetizer seems better for previewing messages that have more than one quote level.
MailHub is powerful, with a dizzying array of preferences and accelerators.