Class D
Name: Regula, "a few unremarkable ores... a great rock in space" ("Star Trek: The Wrath of Khan")
VOY: "Emanations"
Hot, arid, rocky (VOY: "Gravity")
Name: Wehtaan/Paan Mokar’a, "uninhabitable" (ENT: "Cease Fire")

Class H
Oxygen/argon atmosphere (VOY: "Scorpion, Part II")

Class J 
Gas giant (DS9: "Starship Down")

Class K 
Adaptable for humanoid life with pressure domes (TOS: "I, Mudd")

Class L 
Name: Briori planet, now New Earth, oxygen/argon atmosphere (VOY: "The 37's")
Cold, "barely habitable" (DS9: "The Ascent")
"Barely breathable athmosphere" with excess carbon dioxide (DS9: "The Sound of Her Voice")

Class M
Oxygen/nitrogen atmosphere (countless episodes)
Vulcan designation: Minshara Class (ENT: "Strange New World")

Class Y
Class nickname: Demon Class, toxic atmosphere with thermionic radiation; surface temperatures over 500K (VOY: "Demon")
"Toxic atmosphere, sulphuric deserts, no trees, no life at all" (VOY: "Flesh and Blood")

Class 9
Pre-Federation classification. Out of the four gas giants in Sol, none are Class 9. Klingon designation: K'Tal Class (ENT: "Sleeping Dogs")