For those who don't know what the Hfil I'm on about (I hope to Kami there aren't many), Nimbus is the flying cloud owned by Goku.  He doesn't use it much if at all after the beginning of the Namek Saga (I think the last time I saw Goku riding it was when he was on his way to Capsule Corp headquarters, to see Dr. Briefs about his space ship, so he could get to Namek).

Nimbus, or Kinto'un was given to Goku early on in Dragonball, after he and Bulma helped Master Roshi's turtle get back home to the sea (Bulma got her reward of a 3 star dragonball after flashing to Master Roshi-eew!).  

Nimbus can only be ridden by someone with a pure heart (which is why is was of little use to Master Roshi).  Goku had no problem with this, we all know how sweet and innocent he was in Dragonball, even more so than in DBZ (if that is possible).  even Krillen, at first, had, more than a little difficulty riding it before he became firm friends with Goku.  Those who do not have a pure heart will fall straight through what is to them, only a cloud.  

It was Given to Goku (like I said), by Master Roshi, who originally received it from Kami.  Roshi had intended to give Goku the gift of immortality (like him), but after realizing that the "Immortal Phoenix", which would be able to grant him immortality, had had an "unpleasant incident with the birdseed", there was no immortal phoenix, so Master Roshi had to think of something else.

Somehow, Goku gets through more than one cloud.  Goku, being Goku, manages to get a few of them blown to pieces in various battles, so they end up having to be replaced.  Typical.