The most obvious alternative to Sigma's 30/2.8 EX DN is Sony's own E 30/3.5 Macro prime. However, it's not really a direct comparison since Sony's lens is more of a specialist, being a macro lens and all. Other than that, there are not other 30mm primes and you would either need to go wider or longer. Sony branded Carl Zeiss Sonnar 24/1.8 ZA lens is an excellent choice if you're willing to go wider than 30mm, while Sony's E 50/1.8 OSS prime is another good choice for longer FL needs. Otherwise, look for alternatives. If you have Sony's LA-EA1 or LA-EA2 adapter, you can use any of the currently manufactured Alpha mount lenses, including Sony DT 30/2.8 Macro, Sigma 30/1.5 EX DG or Sonny AF 28/2.8. Older Minolta lenses like Minolta AF 28/2.8 might be an option as well.
If you were considering Sony's E 30/3.5 Macro as a general purpose, walk-around lens, forget that idea and go get Sigma 30/2.8 DN instead. Sigma's first entrance into the NEX world is fairly solid and its first native mount NEX lens edges over its chief competitor in pretty much all aspects except macro photography. Faster by almost a full f-stop than Sony, with slight improvements in resolution (specifically around borders) as well as cheaper price point (particularly when we consider the latest price hike on Sony lenses), Sigma offers a very attractive package in this lens. Like any lens, Sigma 30/2.8 EX DN is not without flaws - vignetting and CA might still bother some photographers. What else is there to say? Go get the lens... Sample Images