I have several unused ones as well and often am not too happy when renewal time comes around as while individually it's not that much, renewing 5 domains at around the same time is quite a bit. However, every domain I own has a potential website that I could build around it so therefore, I've kept them all as when time permits, I will go ahead and develop them but there are a couple that perhaps are no longer too profitable and those I might let go off by either selling them or simply letting them expire.