As the moment of this writing (early 2012), Sony E 16/2.8 is the widest rectilinear native lens for the NEX system. An optional Ultra-Wide Converter VLC-ECU1 can convert the E 16/2.8 into an 12mm (18mm full-frame equivalent) ultra wide, but the converter adds additional optical elements and thus reduces image quality somewhat. Additionally, VLC-ECF1 Fisheye Converter can turn E 16/2.8 into a 10mm (16mm full-frame equivalent) fisheye lens.Then there is also SLR Magic HyperPrime 12mm f/1.6 lens, which is effectively a re-badged (with a replaced mount) CCTV lens (pre-announced, but not yet widely available as far as I can tell). That's pretty much it for the moment, although since NEX system has the shortest flange distance among mirrorless cameras, you can easily adapt a wide range on alternative lenses from other mounts, including Voigtlander's 12/5.6 and even a wide range of uber-wide CCTV lenses.
It's good that Sony E 16/2.8 does not have any real competition at this time, as the lens has turned out to be somewhat of a dud. While Sony can be applauded for the pancake concept, which makes the lens more compact and stylish, the optical characteristics of this lens are pretty mediocre. It is also very likely that besides some design constraints that come with the form factor, Sony had to compromise in other areas as well, to make the lens more affordable. Whatever the reason, Sony E 16/2.8 leaves room for improvement. Quite a lot of room actually - resolution of the lens is subpar around corners no matter how much you stop down this lens, distortion is iffy at best, chromatic aberration is also not very well controlled. What else is left of the lens here? It's good that the lens is at least dirt cheap. If you are very picky about quality of your images, particularly at wider apertures, you are probably going to be dissatisfied with this lens.