I was wondering if any Jack Johnson fans could recommend any other singers/groups.
Thanks
Donavon Frankenreiter, G-love and special sauce, Mason Jennings, and Nick Drake. All fantastic, there maybe some more but i cant think right now. Oh and of course Ben Harper
I have to agree with the above... especially Ben Harper -
If you like the song Flake on Brushfire Fairytales thats
Ben Playing the Steel Guitar.
Also you may find Sufjan Stevens appealing if you like the
folk part of Jack.
Also, If you havn't already; you should check "September Sessions"
which is a soundtrack for a surf film by Jack with the same name. 
I really like Switchfoot and Death Cab for Cutie. I love Jack Johnson's music and Switchfoot is sort of similar to his style in my opinion.
If you like Jack Johnson (laid back, but great music) I would recommend Keane or Howie Day. They're a nice way to relax and enjoy good music (as is Jack Johnson).
I'm not too big a fan of Jack Johnson (I like a few of his songs though, specifically Taylor) but Ben Kweller has some similar stuff that is pretty cool.
Keane? really? not that great, nor that similar. Over-rated public school posh boys with an elf of a lead singer. With only one facet. Arrrggghhh
Ben Haper is very similar and very different.
A great musician anyway, just like jack...
I'd say perhaps some ben harper, perhaps some Dave Matthews, although by now may have to be deaf to not have heard of him. John mayer maybe? I might be streching.