The great spider encountered by Frodo and Sam in J.R.R. Tolkien’s The Two Towers is Shelob.
Shelob lives in a pass above Cirith Ungol, near Mordor. Gollum leads Frodo and Sam toward her lair, hoping that the spider will kill them and leave the Ring for him. Shelob attacks Frodo, but Sam uses Sting and the Phial of Galadriel to drive her away.
Shelob is not simply one of the ordinary giant spiders found in Mirkwood. Tolkien describes her as an ancient descendant of Ungoliant, the monstrous spider-like being associated with the earliest history of Arda. That connection gives Shelob a much older and more exceptional place in the legendarium than the spiders Bilbo encounters in The Hobbit.