Unfortunately since Sparrow was acquired by Google [1] they are no longer working on Sparrow.

I still use Sparrow and have zero problems with it, though for some reason I still keep my eye out for new apps to replace it.. I think I just enjoy playing with new features, heh.

There's also the similarly named MailPilot.[2] They're apparently planning on making native apps for every platform. There's currently a public beta for their web app, though there is no free trial; you have to pre-order a year's service (at $60) to gain access, which is a little steep in my opinion!

[1] http://sprw.me/ [2] http://www.mailpilot.co/