I had good luck with the Dri Ducks Poncho. Cheap, light and fairly durable. Since it's breathable it doubles as an extra sleeping layer at night. Last summer I used it in rain every day for 5 straight days. My pack remained bone dry. Just don't use this poncho for bushwhacking.
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.