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.
Other time machines are seen in the movie Dragon Ball Z: Wrath of the Dragon, to allow Tapion to return to Planet Konats in the past, and in the video games Dragon Ball Heroes and Dragon Ball Online.