YouMail

YouMail is a free service which allows people to record different voicemail messages for individual phone numbers calling you, based on caller ID info. Express your undying affection to your significant other, tell your creepy ex to buzz off, or show your sense of humor while remaining professional to your boss. I can see a great use for this product.
The pros:
It's free.
It's easy to use.
It's a good idea.
It could be very popular with people with lots of time on their hands, i.e. teens.
The cons:
Sends you a text message to inform of new voicemail, could get expensive.
Currently only works with Cingular, Verizon and T-Mobile.
No new sign ups are being allowed due to recent popularity.

As soon as it lets me sign up, I'm all in.