Hyberbolic Time Chamber.

The Hyperbolic Time Chamber is a room that is located in Kami's Lookout. The first time anybody has gone into the chamber is in the Piccolo Daimyo saga, Goku entered it then to train with Kami. But that was only in the manga. In the TV show it was introduced in the cell Saga. When you go into the room you feel 10 times the gravity of Earth. One day outside of the chamber is equal to one entire year inside the Hyperbolic Time Chamber.

Goku - Twice
Piccolo - Twice
Vegeta - Twice
Gohan - Once
Future Trunks - Twice
Goten - Once
lil Trunks - Once
Super Buu - Once
Krillin - Once
Gotenks - Once

Goku went in as a little boy and as an adult. He only trained for about 5 months as a little boy because he couldn't stand the extreme heat to extreme coldness. But, as an adult and Super Saiyan, he could take the 10 times gravity and weather conditions. Vegeta went in twice, once, he trained so hard, he became very close to USSJ, and then went back to train once more to surpass Goku. Gohan went in as a kid, and trained so hard, he became a Super Saiyan in that room.

Grown up (future) Trunks went in twice, and achieved USSJ in there, while, his father came very close while training with him. He went in again, to train and become stronger to kill Cell. Goten went in once, while training with lil Trunks. They were training, so they could fight, "the really powerful guy", or we know him as Super Buu. They trained so hard together, but I'll lead on to that with lil Trunks. Lil Trunks went in with Goten, as I said, to train against Super Buu. Now, as they were training in fusion, they achieved something not even Vegeta could. They became Super Saiyan 3 Gotenks. Piccolo lead Super Buu to Goten and lil Trunks, while they prepared, and, fought in the Hyperbolic time chamber. In that time, Super Buu found out it was a trick, and made a portal to get out, leaving Piccolo, Goten, and Trunks in there.

Piccolo trained once in the Cell Saga, only trying to get stronger, but not too much. Another time was when he walked in with Super Buu, to see Goten and lil Trunks could kill Super Buu. However, he got trapped in the room, and had to escape with Gotenks. Now, you may are wondering, Krillin? He went in the room? Well, he did. He went in while the door was open, to tell Goten and lil Trunks to get up for Super Buu. Then, he ran out. That is all the people we get to see go in the Hyperbolic Time Chamber. Was there anyone else that went in? I don't know. That's all I know about the Hyperbolic time chamber.

INSTANT TRANSMISSION