With sales of over 100 million and over 50,000 applications available on them the iPhone and iPod touch are pretty much underrated as a teaching and learning tool.  Jacqui Sharp has put together a great site that has the best apps for students and teachers and even has some uselful lesson ideas for these great little devices.  It is available here and if you or your kids have access to either of Apple's little wonder toys then check it out.