http://www.tunngle.net/en/
Download it, sign up for an account, and find us in the lobby Misc > Gamer-Clans. I'm usually always signed in as gog.bgbrendle (sometimes afk). When you sign up put GOG in your name so we know who you are :)
Tunngle duplicates the conditions of a LAN, so once you're in the same lobby with someone, you're on their LAN. Hamachi tries to do this but is not nearly as good - with this you just log in to a lobby, and anyone who is hosting an IPX or LAN game is visible when you open the game.
It works for any game in gog's collection, but I have a special place in my heart for HOMM :)

damn! I signed up, but forgot to but the [gog] tag before my nick... well, when i'll be connected, anyway, i'll have the same nick than here...

not a big deal :) hopefully I'll see you in there soon

Uh, why does Firefox keep me warning about (in)security problems when I try to register on the website?

There's an even better way than that. Most HoMM players use GameRanger, because it's faster and easier than all the other solutions.
GameRanger is also safer than using these pure LAN emulators, because it doesn't expose your whole computer to strangers on the internet.

bgbrendle> unfortunately won't be able to play for the next two weeks, as some friends come to see me here in Japan...
ScottKevill> thanks for the suggestion about GameRanger. I'll give it a try, too.

I'm going to join here and there as well, I'd love to play with someone other than my best mate. :)