For example, the Bard feature Magical Secrets says:
Choose two spells from any class[...] The chosen spells count as bard spells for you.
I'm looking for a conclusive list of such features. Please note that the Spellcasting feature is not included in this, neither are features that allow you to cast a specific spell a certain number of times per long or short rest.
All official releases.
Motivation: Being able to cast spells such as Green-Flame Blade or Eldritch Blast are normally exclusive to only a small selection of classes. A feature that would allow, say, a Cleric to cast these spells might open up a ton of new options for character builds that are centered around such a spell.