This site offers tools for studying vocabulary. There is also a selection of apps available. What makes this interesting for students is the visuals provided by both images as well as video to give additional cues for learning words.
"A simple Decimal/Roman number converter that will calculate whole numbers from 1 through 3,999,999. Additionally, there is a Quiz mode that quizzes the user - available in either decimal or roman numeral mode."
This free graphic calculator can also serve as a Scientific Calculator, has graphing capabilities, offers a unit converter, and can create a table of functions from values entered. It also has a set of references for formulas.
"CCL4s provides an interface for entering observation data as well as access to all the Content Standards for Mathematics K-12 and the ISTE NETS.S. The collected observation data can be shared via email or with our unique built-in instant meeting. The graphs are very interactive and allow observers to merge different observations to compare and contrast different teachers or the same teacher over time. Using the Timeline feature, observers can track the observed Look-fors through time."
This app is free until April, though know that some content becomes an in-app purchase. Bookabi allows students to create their own storybooks with 2D and 3D characters. They can upload their own photos and pictures plus save and share their final projects.
This is an interesting take on eBooks for pre-school children. Though not free, the app includes audio books, games, and the ability to send a custom report on the students' progress.
This could be the best $.99 spent on an app. Tools 4 Students includes 25 graphic organizers that can be annotated in the app and then shared via email.
This site provides both a social network for your class as well as iOS/Android/Facebook apps to support learning outside of school. Unlike other study apps, this one encourages teachers and students to collaborate and engage via their Study Hall social network