Time Machines build by Bulma, most notably used by Trunks and Cell to travel back in time.
Time Machines built by Bulma which can send a person backwards or forwards in time. However, altering anything in the past does not effect the present, instead creating a new timeline that branches off from the original. Bulma builds the first time machine to send Trunks back in time to save Goku from a heart disease and warn the Z-Fighters about the Androids. Due to several different trips through time, several different timelines are created. Cell and Future Trunks come from different future timelines.