ICQ
ICQ (pronounced "I seek you") is an Internet chat program which is now apparently used for mobile devices as well as desktop/laptop computers.
There are two ICQ ActiveLists devoted to Ars Magica, numbers 54311732 and #78479803. This requires ICQ 99b or better to use; get it from icq.com. Maintainer's note: these have not been verified lately and may no longer be active.
The popularization of fast internet access and smartphones may have made those lists obsolete, though. One may want to try the Official Forum for better results.