Cross Epoch is a DragonBall and One Piece crossover manga that was released in Japan in 2006. It was created by Akira Toriyama and Eiichiro Oda and was featured in the Weekly Shonen Jump. It has now released in the English Shonen Jump, which can be purchased at several locations (store and online).
Here is a quick summary of the storyline...
The main plot is that all the characters are trying to get to a tea party hosted by none other than Shenron. Along the way they encounter obstacles but at the end get there. Its interesting to see how some of the characters are changed physically and in place, such as Vegeta having horns and being a captain of a crew, or Roshi being a cop.