I will use character names so it is easier:
From Agito:
Tsugami/Agito = Employee at Thai restaurant in Ryuki's Episode Final
Ryo/Gills = Punk who gets beaten up by Ouja on the street in Ryuki's Episode Final
Hikawa/ G3-X = in teppan Restaurant in Ryuki's Episode Final
Ozawa = Photography Girl in Blade, eating with Hikawa in Ryuki's Episode Final
Omuro/G3 Mild = eating with Hikawa in Ryuki's Episode Final
Houjo/V1 = Takuma/Centipede Orphenoch in Faiz, eating with Hikawa in Ryuki's Episode Final
Mana = Naomi in Den-o and Decade, Amusement park girl in Ryuki's Episode Final and Police woman in Kiva's King of the Castle in the Demon World
From Ryuki:
Shimada = Smart Lady, and she made a guest apperance in Den-o too
Megumi = Shura (the Zect Girl) in Kabuto's God Speed Love
Tezuka/ Kr Raia = Nigo in THE FIRST and THE NEXT
Kanzaki = Worm/Zectropper
Sanako/Yui's Grandmother = cameo in Faiz's Paradise Lost
From Faiz:
Mari = Mio in Kiva
Kusaka = guest apperance in Kabuto, was in KrG and Decade as Kaixa again
Kaido = security guard in Blade's Missing Aces, Soldier in Kr G
Murakami/Rose Orphenoch = Violinist guy in Kiva, guest appearance in Blade's Missing Aces
Kiba/Horse Orphenoch= guest appearance with Murakami in Blade's Missing Aces
Rabbit Orphenoch = Punchhopper in Kabuto
From Blade:
Hajime/Chalice = Suzuki's (Onedera Yuusuke) police daddy in Climax Deka
Teen Amane from Missing Aces = Kotomi (Chiharu's friend) in THE NEXT
From Kabuto:
Yaguruma = Kickhopper in Decade and Imagin in Ore Tanjou
Kageyama = Punchhopper in Decade and Imagin in Ore Tanjou
Kagami Riku = Guys who gets killed in the first scene of THE FIRST
Daisuke/Drake = V3 in THE NEXT
Hongo Takeshi = Hongo Takeshi in THE FIRST and THE NEXT
Gatack Zecter in HBV = Urataros in Den-o
From Den-o:
Momo, Ura, Kin, Ryuu = all made guest appearance in human form in Kiva's King of the Castle in the Demon World
All the Imagins = in Decade
New Den-o = in Decade
Kid Ryotaro = in Decade
Kohana = in last episode of Kiva and in Decade
Suzuki (from Climax Deka)= Yuusuke/Kuuga in Decade
Miura = Cop in Kiva's King of the Castle in the Demon World
Soccer Kid = Ramon/Basshaa in Kiva and Chou Den-o
From Kiva:
Wataru = Wataru in Decade
Kivat II = Kivat in Decade
Spider Fangire = Spider Fangire in Decade
Otoya = will be in Decade
From Decade:
Oji-chan/Natsumi's Grandfather = got killed with Kagami Riku in THE FIRST