Alex Eames, over at RasPiO, discovered that his GPIO reference PCB, known as the Portsplus did not fit the new Raspberry Pi 3B+ because of the new Power-over-Ethernet header. To get round the problem, he has rejigged and redesigned the board to avoid the header and make it compatible with every 40-pin Pi. For the first week or so, he is offering the new Portsplus for free (just pay £2/£3 shipping) from his website. To take advantage of the offer, head over to RasPiO.
These are great little boards which are thin enough that they can be placed over the GPIO pins and help you to find the right pin without all the tedious counting. Great for prototyping!