iBird Explorer PRO. Mitch Waite Group, 2011 ($2.99). For iPhone/iPad. This top-rated app for bird lovers (right) features 924 North American and Hawaiian birds and their songs.

LeafSnap. Columbia University, University of Maryland and Smithsonian Institution, 2011 (free). For iPhone/iPad. Snap a picture of almost any leaf, and this cool app will help you identify it by bringing up images and names of possible matches.

Particle Zoo. Richard Burgess, 2011 (free). For iPhone/iPad. Mainly for kids but also a handy primer for adults, this app describes all the particles and antiparticles—including those that have not yet been discovered.

Planets. QContinuum, 2012 (free). For iPhone/iPad. An addictive real-time map of the sky showing when the planets and moon are, or will be, visible, plus great data on each planet's mass, orbit and moons.