You can play online using Aleph One. Uhm... I've never played it myself so I'm not of much help. There's a chat room on AIM where you can hook up with other players. It probably called "marathon" or something like that... Oh and you have to download the nightly builds of Aleph One for it to work.
http://source.bungie.org/
http://alephone.falkoa.de/
: I've never got the chance to play the multiplayer option on Marathon, sadly.
: It's something I've wanted to be able to experience for years now. Have
: there been any updates that allow better access to online Marathon play,
: such as maybe LAN? I desperately need to be able to play online.