Oblique Strategies (subtitled Over One Hundred Worthwhile Dilemmas) is a deck of 7-by-9-centimetre (2.8 in × 3.5 in) printed cards in a black container box, created by Brian Eno and Peter Schmidt and first published in 1975. Each card offers a challenging constraint intended to help artists (particularly musicians) break creative blocks by encouraging lateral thinking. Wikipedia
The entire story of Oblique Strategies, with the content of all the cards, exhaustive history and commentary, is documented in a website widely acknowledged as the authoritative source, put together by musician and educator Gregory Alan Taylor.
You can purchase a pack of the cards directly from the Eno Shop.