Although it won’t grow new hair on a bald scalp, Rogaine foam is a very good product for maintaining hair and I use it almost daily.  For best results, you are supposed to use it twice a day, but I never use it more than once a day. Unlike the old liquid version, the foam does not cause a mess or trickle down your face or back of neck, dries rapidly, and actually holds your hair like a weak gel.  Amazon currently has one can for sale at only $10 (usual price = $30) with limited quantities available:

Edit: Seems like the product is sold out at that price. Still some decent discounts for 3 month supplies.