Synonyms for Apropos (same or very similar meaning)
WordNet sense 1 (by the way):
incidentallyWordNet sense 2 (at an opportune time):
seasonably, timely, well-timedWordNet sense 3 (being of striking appropriateness and pertinence):
apposite, appropriate, apt, pertinent, appropriateFrom the ODE community, based on WordNetadd/edit