Requires smelting a Plastic Ball.
Plastic Goo is whats left when you heat up plastic - naturally ! So, what use is it ? Well, it comes in use for repairing tools once you use it to make Duct Tape. It can also be used as a replacement for slimeballs when making a Sticky Piston. There is also the Plasticgoo Block which is really useful for mob traps, so not quite as useless as you thought. It can be smelted in the stone furnace, the Microwave Oven,or either of the two Extraction Furnaces.
Of course, you can try picking up some lava with a plastic bucket, which will result in the bucket melting and you ending up with a sicky mess of goo in your hands. Doing this will use up a source block of lava.