Gizmo is a useful site which offers the ability to send free SMS messages to quite a few different networks in dozens of countries (including Optus in Australia). No registration required and the simple uncluttered interface is a model of functionality.
Great for sending messages to loved ones and, if you pick up a prepaid SIM overseas, for giving friends a free, instantaneous, means of contacting you.
The website is associated with Gizmo5, a competitor to Skype (the service which has become synonymous with cheap voice-over-IP telephony). Like Skype, Gizmo5 can be used from a computer or for slightly higher rates downloaded to your mobile. Many internet cafes have embraced on the opportunity to offer cheap international calls, although for security purposes running a portable version of Skype from a thumb drive/USB key is preferable. Gizmo doesn’t currently offer a portable version, but GizmoCall is perhaps even better – it allows calls to be made over the service from within a web browser.