The Hyperbolic Time Chamber
The Room of Time and Spirit was first introduced in Dragon Ball. Although this was only in the Anime, Goku went in with Kami for training. After 5 months Goku couldnt stand the 10 times gravity or extreme heat to extreme cold. In the Manga the First time we see the Room of Time and Spirit is in the Cell Saga. One day in the Room is equal to 1 year of training. This is why it is so great. In side the room, time is slowed down giving the trainer more training time to fit in a day. when you come out you are aged 1 year. We see Gohan age a great deal after training in the room. You may ask why they dont train in the room all the time. Well its simple, One person can only go in the room for 2 days ever. If they go in again they will be locked in nothingness for all eternity.
The people that we have seen in the room are:
Goku - went in twice
Piccolo - went intwice
Vegeta - went intwice
Gohan - went in once
Future Trunks - went in twice
Goten - went in once
Chibi Trunks - went in once
Super Buu - went in once
Krillin - went in once
Gotenks - went in once