Here are the ones I went with - the links are for iTunes just because I have an iPhone but I think most of them are also on Android, and because a list of links looks a little boring I have of course added one of my relevant photos!
Bee Count - Friends of The Earth's Great British Bee Count - ID and log sightings between May and June.
BirdSongID - £3.99, record the bird and the Automatic Recognition identifies it.
BirdTrack - This is from the BTO and allows you to log your sightings and see what others have seen.
Stellarium - £2.99 Point your phone at the sky and identify the constellations, stars and planets.
And of course I already have the 30 Days Wild app! Please let me know of any nature apps you wouldn't be without.