WD-40 is a penetrating oil that is useful for helping free rusted parts and stop squeaking doors. You can purchase a can of it at any hardware store, but you can also make your own in a pinch if you have access to acetone and automatic transmission fluid made from dextron/mercron.
One Instructables user points out that this DIY recipe worked better than the commercial product when having to remove a rusted brake rotor from an axle. To make your own, just mix up a 50/50 batch in a cheap spray bottle and shake well before using. You probably already have acetone around the house (paint thinner or nail polish remover), but the automatic transmission fluid might have to be procured at a local auto parts store.
Home-made pentrating oil [Instructables]