P3X-888 is the Tau'ri designation for the homeworld of a race known as the Unas, the first race that the Goa'uld ever took as hosts. This world is also the only planet where Goa'uld larvae have been discovered in 'the wild', and also the only known world where Goa'uld were present in the fossil record, leading to Daniel Jackson to speculate that P3X-888 was also the Goa'uld's original homeworld. (SG1: "The First Ones")
"Wild" Goa'uld still prowl the rivers and streams of the planet, from which they once leapt to take Unas as their first hosts. From there, they discovered the workings of the Stargate and abandoned the planet and spread amongst the stars — though Goa'uld and Unas still reside there in a hostile tension.
It should be noted, there is no naquadah in the blood of these Goa'uld. It's possible these Goa'uld are not entirely malevolent and may not be entirely sentient either, as the Goa'ulds that took SG team members as hosts did not seem to retain full control and did not seem to have any agenda other than the goal of inhabiting their host.
The number 888, perhaps ironically, is a number associated with good fortune in real life.
Behind the scenes
Stargate SG-1: The DVD Collection issue 26 gives an alternate address for this planet (
), while the episode Endgame gives the address 
. As Stargate SG-1: The Ultimate Visual Guide was produced at a later date, it can be assumed that these addresses have been overwritten.
The moons of P3X-888.