Hercules (Herakles), like some of the other major heroes, went to the Underworld. Unlike the others, he seems to have repeated his visit while still alive. How many times did Hercules actually go to the Underworld before death?
Hercules' Journies Into the Underworld
It is not entirely clear how many times Hercules went to the Underworld. As the 12th Labor Eurystheus assigned for Hercules' penance, Hercules was to fetch the hound of Hades, Cerberus (usually shown with 3 heads). Hercules was initiated into the Eleusinian mysteries in order to engage in this act, so he wouldn't have descended to the Underworld before this labor, at least within the logic of Greco-Roman mythology. While he was there or, possibly, on another occasion, Hercules saw his friend Theseus and observed that he was in need of rescuing. Since Hercules returned to the land of the living immediately after rescuing Theseus, and no other purpose is assigned Hercules' visit at the time, other than borrowing Cerberus, it makes sense to see this as one and the same visit to the Underworld.
The other occasion when Hercules may have descended to the Underworld is the rescue of Alcestis by wrestling her from Thanatos (Death). This rescue may or may not have occurred in the Underworld. Since Thanatos had already taken Alcestis (the brave woman who was willing to sacrifice herself so that her husband, Admetus, might live), to me it seems more likely that she was in the land of the dead, and so I take this as a second trip to the Underworld. However, Thanatos and Alcestis may have been above ground.