It seems unbelievable, but you can actually get the pitch of a bass guitar out of a ukulele. This is thanks to the polyurethane strings. The first people to release a bass ukulele were Road Toad. However, it was expensive. More recently, Road Toad have hooked up with Kala to produce the much more affordable Kala UBass. This is a standard baritone Kala ukulele adapted to be able to handle the much thicker strings.
I haven't tried one of these ukuleles but I'm dying to give them a go.
One thing to note, if you're buying one get a bass amp at the same time. The acoustic volume of them isn't, apparently, sufficient.
A Kala U-Bass demonstrated by RISA ukuleles.