• Project Noah (free) "the best way to share your wildlife encounters and help document our planet's biodiversity." Get your students involved in becoming citizen scientists
• Leaf Snap (free) "the first in a series of electronic field guides being developed by researchers from Columbia University, the University of Maryland, and the Smithsonian Institution. This free mobile app uses visual recognition software to help identify tree species from photographs of their leaves"
• Trees Pro HD-Nature Mobile (free)  guide to the most common North American and European trees on your iPhone and iPad. With the built-in quiz you will also learn quickly to distinguish between different species. * Please note: You get 40 common species in this version. In addition, you will have the option to buy more than 160 species in In-App sales. 
• StarWalk ($2.99) interactive astronomy guide ALSO: Check out The Night Sky which is very similar and is on sale for .99 as of 2/20/12
• The Elements: A Visual Exploration ($13.99-iPad ONLY) 3D periodic table--beautiful way to learn the elements! 
• Coaster Physics (.99) "What are the laws of Physics governing the motion of a roller coaster? How are speed and acceleration related? How does the g-force change at different points along the track? Design and ride your very own realistic roller coaster, and see how quantities like speed, acceleration, energy and g-force change as you ride along the track."
• TinkerBox HD (free-iPad ONLY) "TinkerBox is a fun, free-to-play physics puzzle game. While it is full of interesting science facts and teaches basic engineering concepts. Take the tools in your hands to explore your creativity and imagination with Invent mode. Build outrageous machines, share them with your friends, or download popular inventions online athttp://www.tinkerboxnews.com."
• iNaturalist
• Science 360
• Interactive Science Glossary
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Haven't tried this one, but it's getting high reviews: 

• Bobo Explores Light ($4.99) Designed for children ages 8-12, Bobo Explores Light introduces kids to 21 light-related topics, like lasers, reflection, refraction, photosynthesis, bioluminescence, aurora borealis and the human eye. In each section, readers engage in hands-on experimentation to build a deeper understanding of science concepts. Supplementary videos, articles, animations and trivia are designed to meet children at whatever learning level they feel most comfortable.
• Simple Physics (free)