LiSA, the Liquid Information Speaking Assistant tells you who your incoming messages are from and announces them as-and-when-they-arrive, not just when you log on to check your messages.
For example: "You've got a message from an important client", "You've got a reply from your lover" all with a real human voice (not computer synthesized speech).
How can LiSA know which messages are interesting to you? You tell her: LiSA can be customized to announce all incoming messages, announce only messages from people in your Address Book, or announce messages which are only from the user's Address Book unless they are replies or the words "important" or "urgent" are in the message Subject. You can also specify that she is to ignore emails from specified addresses all the time. She can talk a lot or a little - it's all up to you. She can even announce the time.