Alright, so, I'll list all the arcs available out there and say if it's translated or not, also say where and if you can download it.

Anime has 18 arcs in total:

Higurashi no Naku Koro ni: Onikakushi-hen, Watanagashi-hen, Tatarigoroshi-hen, Himatsubushi-hen, Meakashi-hen, Tsumihoroboshi-hen

Gaiden: Nekogoroshi-hen

Kai: Yakusamashi-hen (anime-only), Minagoroshi-hen, Matsuribayashi-hen

Ura Higu/Kai Specials: No really arcs to see here, just a compilation of funny shorts

Rei: Hajisarashi-hen, Saikoroshi-hen, Hirukowashi-hen

Kira (anime-only, except for Batsukoishi-hen): Batsukoishi-hen, Ayakashisenshi-hen, Musubienishi-hen, Yumeutsushi-hen

Kaku: Higurashi Outbreak

Note: All anime OVAs and season are translated and can be easily found, except for Ura Higu/Kai Specials, that seems to only have 14 episodes translated (some of the titles aren't though).

The entire Visual Novel series has 23 arcs (24 if you count Miotsukushi-hen Omote and Ura):

Original VNs: Onikakushi-hen, Watanagashi-hen, Tatarigoroshi-hen, Himatsubushi-hen, Meakashi-hen, Tsumihoroboshi-hen, Minagoroshi-hen, Matsuribayashi-hen (8 arcs - Official translation is on works by MangaGamer on Hō bundle, but you can easily find fan translations)

Matsuri: 7 first main arcs, Taraimawashi-hen, Tsukiotoshi-hen, Miotsukushi-hen Omote (10 arcs - As far as I know, there's no translation available and I quite doubt there will be)

Rei (fan-disc): Saikoroshi-hen, Batsukoishi-hen, Hirukowashi-hen (3 arcs - Official translation is on works by MangaGamer on Hō bundle, but you can easily find fan translations)

Kizuna VNs (4 in total): All main arcs and Rei (except for Batsukoishi-hen) (alternate Hirukowashi-hen version), Someutsushi-hen, Kagebōshi-hen, Tokihogushi-hen, Yoigoshi-hen (alternate version from the original manga, though I don't remember where I got this information), Miotsukushi-hen Ura, Kotohogushi-hen (16 arcs - No official translation, but it's pretty easy to find a translated romhack if you ask)

Sui: All arcs presented on Matsuri and Kizuna (both Miotsukushi-hen Omote and Ura are there, Hirukowashi-hen is altered like in Kizuna), Hajisarashi-hen (this time part of the actual game) (19/20 arcs - As far as I know, no translations available so far)

Original Hō: All main arcs and Rei, Higurashi Outbreak, Kamikashimashi-hen and Hinamizawa Teiryūjo (14 arcs - Being translated by MangaGamer on their website and Steam. So far only done up to Minagoroshi-hen, Matsuribayashi-hen's thumbnail has been revealed in MangaGamer's Twitter)

Entergram Hō: All arcs presented on Sui (both Miotsukushi-hen Omote and Ura are there), original Hō and a "new version of Batsukoishi-hen" (23/24 arcs - No translations available, mostly because of the lenght of the game - 120,000+ dialogues. 07th Expansion has already tried and probably keeps on trying to find someone to translate it. Most likely it'll be translated by Witch Hunt, but if so, that will still take a while)

Note: Some specific arcs are translated by fans, just not the games.

The entire manga series has 22 arcs (?? - I'm not sure about that since it's quite difficult to know exactly, but I'll say the ones I know)

Main arcs and Rei's first (Onikakushi-hen, Watanagashi-hen, Tatarigoroshi-hen, Himatsubushi-hen, Meakashi-hen, Tsumihoroboshi-hen, Minagoroshi-hen, Matsuribayashi-hen, Saikoroshi-hen)

"Kai": Utsutsukowashi-hen (uncontinued after the 3 first chapters)

Hinageshi-hen

Kataribanashi-hen (this one has about 33 mini-arcs, so I'd rather call those "chapters". It's a compilation of fan stories and you can find some separated ones in this wiki, like Usomabushi-hen)

Kokoroiyashi-hen (in the end of this manga, there is an advertisement of an arc named "Yuriutsukushi-hen", but I couldn't really find it anywhere)

Onisarashi-hen (apparently there's a bonus(?) arc in this one: Urakaeshi-hen)

Yoigoshi-hen

Jan: Tsubamegaeshi-hen

Genten: Hinamizawa Teiryūjo

Note: Most (or likely all) of the manga above are fan-translated and you can find them (quite) easily.

Lights Novels: Nekogoroshi-hen, Kuradashi-hen, Kuradashi-hen Zoku (though I don't seem to find these anywhere)

Matsuri Original Soundtrack: Hajisarashi-hen (though I don't seem to find this anywhere)

You also have some Live-Actions and a Stage Play that doesn't actually include new arcs, but they are:

Higurashi no Naku Koro ni (Shrill Cries of Summer) - Mostly Onikakushi-hen

Higurashi no Naku Koro ni Chikai (Shrill Cries of Summer: Reshuffle) - Mostly Tsumihoroboshi-hen

Higurashi no Naku Koro ni (When the Cicadas Cry) - TV Series, it adapted Onikakushi-hen, Watanagashi-hen and Tatarigoroshi-hen

Higurashi no Naku Koro ni -Ryū/Mei- - Stage play, Ryū adapted Watanagashi-hen and Mei adapted Meakashi-hen

Note: The two live-action are quite difficult to find, but I'm quite sure there are translated versions out there. The TV Series and the stage play aren't translated (as far as I know).

Hopefully I helped ^^

Edit: Forgot to mention, Kaku might have Kamikashimashi-hen's adaptation, since the Higurashi Outbreak animators said it would have a different ending from the one released on Hō.

Edit²: Typo and corrections.

Edit³: Additional informations.