Send instant popup messages. Very low resource consumption. There are many ways to communicate on a network.
Email is widely popular but mostly quite slow and requiring people to check their account.
Nearly all of these requires an unrestricted internet connection and often they eat up a lot of resources when activated. The messenger sits discretely in your systemtray and lets you send and receive instant popup messages, chat and send files quickly. OfficeNet Messenger is released as uncrippled shareware.