Gasket goo is possibly one of the most useful substances ever created by evil scientists. It comes in several forms, from several manufacturers, with varying applications. It's the duct tape of automotive engineering. Intended to form a gas-tight seal between pieces of metal, its usefulness knows few bounds. It's also known as gasket sealant.
There are so many manufacturers of gasket goo that I'm not even going to try to list them all here. In the US there are several brands calling themselves 'Liquid Gasket'; I've not had direct experience with any of these myself. They cost about $5 a tube (£3 or £4 in the UK). There's a very famous brand called Hylomar High Performance Gasket that I should think is available everywhere.
Blue gasket goo is remarkable in that it does not dry. It can spend twenty years inside an engine, and it will not dry. It won't even go black. It comes in a squeeze tube, and is applied to one or the other mating surface. The solvent takes a few minutes to evaporate, leaving a blue goo that won't ever dry out. It's particularly useful for repairing small breaks in damaged gaskets and for holding gaskets in place during assembly. According to the Hylomar website it's made of a polyester polyol compound.
Yellow gasket goo has the unique property of preventing corrosion at joins of dissimilar metals. It is particularly useful for joining a stainless steel exhaust manifold to an aluminium engine, for example.
Red gasket goo is very useful because it doesn't attack rubber or plastics. This is useful because you can coat the piston from your brake caliper with it, making it much easier to insert without damaging, and less likely to seize or bind, especially over winter.
If you know of any more forms of gasket goo, please /msg me.