I'm starting to wonder how I ever did research without the Reunion app and my complete family tree with sources on my iPhone. I love being able to consult a family group or doublecheck a source for a date without bringing my laptop and balancing it on my knees while I sit at a microfilm reader.
So I was happy to find a new Web site devoted to genealogical applications for mobile devices: mobilegenealogy.com. It came in handy at the local LDS library when someone noticed me using my iPhone, but his smartphone was an Android and I had no idea what apps were out there. mobilegenealogy.com covers all mobile devices and their info is up-to-the-minute.
Because Reunion for iPhone ($14.99 from Leicester Productions) does such a great job with its mobile app, I haven't really needed to scope out the many family tree viewer apps out there. It's pricey, but WEP (worth every penny).
I do have Ancestry's Tree to Go (free from Ancestry.com), which provides access to the family trees you've uploaded to their site, but I find it slow and clunky.