I

yahoo messenger (and most of the other messengers out there). Its really one of the best alternative ways to get in contact with a person ASAP, you gotta request - bam it pops up on the other person's desktop, much better than email. You don't incur long distance phone bill charges. And if you are away or even signed off, it logs the message until you return.
If you need to contact someone or someone needs to contact you fast,

the yahoo messenger and there it is right in front of them. Its a great invention.