The easiest way to check out all the services that VoxOx supports would be to look at the image below, which offers a great overview of what VoxOx is about.
Other key features of VoxOx include a unified meta address book which contains contacts from all the various services and networks; VoIP telephony with two initial hours of free talk time (Mashable readers get 1200 minutes, more on that below), inbound/outbound calls, voicemail, call forwarding, inbound faxing, two-way texting, and even a landline replacement (if you're so inclined). It supports Windows and Mac based machines, and support for Ubuntu, Debian and mobile is coming soon.
It's hard to find what's missing here; it's like Trillian and Skype in one with some sugar on top. Of course, the application itself needs to prove that it's reliable, stable and at least as simple to use as Skype, or it'll be hard to create converts from that immensely popular desktop VoIP solution.