Zandronum is a free open-source advanced multiplayer sourceport capable of running various games such as Doom, Doom II, Final Doom, Heretic, Hexen, and as well as others online. It originated from Skulltag, which was based on ZDoom and its counterpart, GZDoom.
Hard Doom is a replacement/enhancement mod that replaces all monsters in the classic Doom games with versions that are similar in role, but more dangerous. In general, Hard-Doom is geared towards players who want a more difficult, genuine experience, and it has a variety of visual effects that are more subtle and don't block your view of the action. Over the years since its initial release, it has been meticulously balanced and rebalanced, with new features and effects added to make this a great mod to play through the popular megawads with, without changing Doom's flavor or core gameplay.
I'm surprised to see someone uploaded this here! I actually plan on making a ModDB page for Hard Doom once I get the next release out. :)
Awesome! When you do make sure to tell me and I'll move this to your page.