The Kala UBass is the size of a baritone ukulele but has the range of a bass guitar i.e. it is tuned EADG in the same octave as a standard bass guitar. It achieves this through polyurethane strings.
The original bass ukulele was the Road Toad. The downside is that it was very expensive - out of the range of most ukulele players who might be curious about it. Then they hooked up with Kala who used Road Toad's technology and adapted a standard baritone ukulele to produce a cheaper bass ukulele. The Kala U-bass is in a range that makes it affordable to someone who wants to include bass in their ukulele group but wants to keep the aesthetic.
One thing to keep in mind if you do buy a Kala UBass is that you need to get an amp at the same time. Polyurethane strings don't produce much volume on their own and need to be amplified.
It comes in two versions: fretted and fretless. If you're used to playing fretted instruments, it takes some getting used to a fretless instrument. But it does give you that distinctive sound of a stand-up bass.
The folks from RISA demonstrate the UBass.
On eBay US
Kala U Bass Polyfoam Hard Gig Case
On eBay UK
Construction: Solid mahogany top, back and sides.
Fretboard: Rosewood. 16 frets (12 to body)
Tuners: Custom Hipshot