During research trips, I often use Evernote and Google Maps/Google Earth. These apps aren't genealogy-specific, but I've found them quite useful for accessing needed notes and locating elusive cemeteries.In addition, I also use:Families Google TranslateAncestry.com's new app
I haven't had my smart phone very long but have been playing with/using several apps that I think are useful for genealogy. I do similar types of research (history/archaeology) for work and these apps come in handy (well, except for youtube and Google Reader while I'm on the clock :). In addition to Evernote, Google Earth/Maps, and ancestry.com that Jo mentioned, I have Dropbox, Google Books, Youtube, Photoshop Express, Amazon Kindle, Camscanner, Google Reader (for catching up on blogs when away from the computer). I also have the Mozy app (my cloud of choice) so I can access all files on my hard drive.