SkipCount
A reminder app that works in the background. Countdown to any event and the app will remind you when it arrives, or give you an up-to-the-second countdown. Free

SlideShark
Show PowerPoint presentations right on the iPad — which can of course be ported to projectors with the right dongle — while preserving the same text, animations, and graphics. Free

Silent Light
Silent Light shows the decibel level in your learning space with an easy to understand traffic light system, which you can share on your projector or TV. Set optimum sound level goals to help students manage their own learning. $4

Socrative
A smart clicker and student response app that captures student results in real-time. Interact with the data to further student understanding in the moment, and review the reports to prepare for future classes. It will even aggregate and grade your pre-made activities. Free

GradeBook Pro
Record scores, attendance, behavior, and student performance in this app, which was designed with professional educators in mind. There are plenty of bells and whistles, like exporting options and data analysis. $20

Essay Grader
Designed to help standardize the essay grading process, this app lets teachers organize papers and select from lists of pre-written comments (or add their own) and guides them through each part of the essay, from thesis to conclusion. $6