  This is mainly an expository text on the Haagerup property for countable
groupoids equipped with a quasi-invariant measure, aiming to complete an
article of Jolissaint devoted to the study of this property for probability
measure preserving countable equivalence relations. We show that our definition
is equivalent to the one given by Ueda in terms of the associated inclusion of
von Neumann algebras. It makes obvious the fact that treeability implies the
Haagerup property for such groupoids. For the sake of completeness, we also
describe, or recall, the connections with amenability and Kazhdan property (T).
