I'll second Ka`au Crater Boys, Troy Fernandez is a wizard. Troy also recorded some great ukulele with Palolo.
Ohta San has some nice melodic jazzy music and Herb Jr follows in his footsteps more or less, but Herb Jr plays more Hawaiian tunes. Look for him recording with Keoki Kahumoku and David Kamakahi. David's U Lili `E (from the Ohana CD) is a classic uke tune that he recorded when he was only 17. His dad is the legendary Dennis Kamakahi.
Also check out Brian Tolentino, he's got a great CD called "Ka `Ukulele Lele" and has some nice chops - lots of great ukulele. He plays in Waikiki when he's not on tour.
Moe Keale's "Honi" from his Imagine CD has some nice uke in a different style. Moe played with The Sons of Hawaii so listen to them for some old traditional Hawaiian music and you'll hear the ukes in there, although they aren't featured as much as they are in much of the new music.
That's just some of the Hawaiian stuff. There are many more great players, James Hill, Lyle Ritz, Dan Saywer, Janet Klein, Craig Robertson - hey, you can catch Craig live at the Sky Bar in Somerville!