A couple of weeks ago, I was playing Hexen, Heretic and Descent 2 with a couple of friends over Hamachi, which was pretty damn awesome. I also had a Teamspeak server running through it, which made it even more awesome, so I'm wondering if maybe some of you here would like to join for whatever games.
The server is called Br00talbro and the password is lefse.
You can download Hamachi from this location.
For Hexen and Heretic we used , which worked quite well, even though we had to fiddle around with some bat files to be able to play properly. We also managed to save and load without problems, so coop games can be saved and continued later (just remember to join the correct order). It's also possible to use[url=http://www.zdaemon.org/]ZDaemon for Doom at least.
For Descent 2 we used DXX Rebirth, but D2X-XL can also be used, and I’m up for multiplayer in any of the Descent games.
If anybody is interested in Rise of the Triad, there is a source port called WinROTT, which can be used for both singleplayer and multiplayer.
Also, feel free to come with suggestions and such.
EDIT (for MOAR games):
Duke Nukem 3D is awesome in so many ways, so here's a source port for that.
Freespace is also very awesome, and FSOpen can enhance it even further.

Hmm, is there coop in Freespace 2?

Any chance for Duke?

According to the manual, yes.
EDIT:
Forgot Duke, but absolutely! I'll add a link to a source port in the OP.

Well I'm interested in freespace 2, or will be when I stop feeling like shit

Descent 2 Rebirth at around 15:00 GMT+1 tomorrow anyone?
Last night I talked with some dude on the Hamachi chat, who had googled for Descent 2 multiplayer and found this thread, so with that suggestion I think it might be something to arrange.