All of these are free, and highly recommended!
Greenshot--Screenshot tool for Windows. Take pictures of all, or part of any computer screen.
Screenr--Screenr stands out because of how incredibly easy it is to record a screencast and share it with your friends.
Screencast-o-matic--Screencast-O-Matic is the original online screen recorder for one-click recording from your browser on Windows, Mac, or Linux with no install for FREE!
Jing--Screenshot tool for Mac and Windows
Prezi--Make your presentations zoom
Animoto--Create stunning video slide shows
LibreOffice (for Windows) (for Mac)--LibreOffice is the power-packed free, libre and open source personal productivity suite for Windows, Macintosh and GNU/Linux, that gives you six feature-rich applications for all your document production and data processing needs: Writer, Calc, Impress, Draw, Math and Base.
Storybird--Collaborative storytelling. Storybirds are short, visual stories that you make with family and friends to share and (soon) print.
bubblus--Bubbl.us is a simple and free web application that lets you brainstorm online. You can: Create colourful mind maps online, Share and work with friends, Embed your mind map in your blog or website, Email and print your mind map, Save your mind map as an image.
Wordle--Wordle is a toy for generating “word clouds” from text that you provide. The clouds give greater prominence to words that appear more frequently in the source text. You can tweak your clouds with different fonts, layouts, and color schemes. The images you create with Wordle are yours to use however you like. You can print them out, or save them to the Wordle gallery to share with your friends.
Communication and Collaboration Tools
Primary Pad--A web based word processor designed for schools, allowing for real time collaborative writing between students and teachers.
Wallwisher--Wallwisher is an online notice board maker, a new way for you and your students to communicate!
SurveyMonkey--Create free and easy online surveys using this tool.