Until something better or easier to use comes along, it looks like
eBay Skype has most of the market in the accessible VoIP-from-your-computer. It's always been easy to see if someone else is online from your copy of Skype, but advertising that to another place (like somewhere on the web) has been difficult. There have been various plugins or helper applications which need to be downloaded, but finally Skype released their Web Presence API... this means that with a single line:
This could be really useful for us, since a number of our headteachers and networked learning communities use Skype as a really useful way - it would be a very easy thing to create a Moodle HTML block which showed which of the groups were on - so this could be visible to anyone from anywhere, without needing Skype.
If none of this makes sense then there's a Flash video (with audio) available from Skype which explains it...