Fire up any of Bungie's three Marathons under AlephOne and play online at Mariusnet.com.
Now all three Marathon games are free downloads from http://source.bungie.org/ . Click "Download" and get the three original scenarios, get AlephOne, and get hi-res textures (optional). It is classic goodness from classic Bungie. I love classic stuff. It won't be like Xbox. It'll be classic, and updated.
To make Marathon games work in OS X or current Windows OS, download AlephOne from the same link, then do what I did.
To play Marathon, here's what you do (that is to say, here's what I did):
1) I Downloaded Marathon 1, 2, and 3 (Marathon, Durandal, and Infinity) from above link.
a) I then expanded the compressed files into their folders full of game files.
2) I then Downloaded AlephOne and expanded it.
3) Copied all the AlephOne files into each of the three Marathon game folders I had so they each could be played in OS X (or Windows).
I also optionally downloaded the hi-res textures (see link above) and followed their installation instructions.
To play Marathon online
Simply choose either "Gather-" or "Join Network Game" and you'll be hosting or seeing games on mariusnet. You can log in automatically as a guest, or you can go to Mariusnet.com and register a free account. I had an account already from ages ago when b.net died.
Mariusnet.com and its forum is, by the way, the current thriving hub for all things Myth-related as well. Playmyth.net is no more. And that's good.
'Sgreat!