Well, i thought of listing them here, since the wiki omits 3 of them.

Demon Lord Belial:

1) Beliar's Trident (you get it by defeating the boss). The only one listed in game wiki.
2) Demon Storm Scythes. You break all breakable objects in one of the town (outside in open air) environments and a hidden chests with those in, appears.
3) Hell Maul. You get it from a chest in a very difficult to reach area, one you reach via underwater swimming. In original 2 i think they were dropped by the DLC's second boss in the hideout.
4) Ice Reaver. Well, i somehow found this one too, isn't listed in the wiki, but i cannot remember where from, exactly.

Argul's Tomb:

1) Frostforged Twins. They are listed in the wiki and you get them from the final boss.

Abyssal Forge:

1) Axe. You get this from going into an area AFTER getting the protective item. It is next to two other chests, in the middle of a lake made out of corrosive liquid.
2) Hammer. You get this and it is not miss-able.

Also DLCs have chests that give skills. Make sure to miss nothing. In Abyssal Forge it is in the area you start (enter the door at the point you begin and solve the puzzles, loot everything), in Argul's Tomb at the end of the cave. I haven't found a skill chest in Belial's though, it might as well be hidden (some skill chests are hidden), but maybe not, argul's chest gives SIX skill points!