A tough choice but ...
Jing or Screencastle - Jing was my favourite screencasting tool - for quality and ease of use, but the 'one-click', no-sign-in simplicity of Screencastle means it's just about taken over. Ooh, but then Jing is so great for grabbing still screenshots... I can't decide.
Nicetranslator - as a language teacher I have come to count on this tool. This is how I want my technology: simple, clever, uncluttered.
Quizlet - my old love, but still wonderful. Quizlet's young creator never rests on his laurels and is always looking for new and imaginative twists to enliven the study of online flashcards.
iGoogle - How did I manage without it? Especially to manage Twitter, Gmail and Google Reader.
Clevr - It rarely works perfectly, but is usually good enough - and makes great panorama shots which can be used to make a virtual tour or induction matrials. The capacity to add hotspots make this an underrated tool imho.
Jottit - As with NiceTranslator - I love its simplicity and flexibility. Get content online in seconds - and so uncluttered.
SimplyBox - OK so I use Delicious more, but the interface on SimplyBox and the neat toolbar makes clipping and sharing a real pleasure.
Classtools - Not one tool, but a set. I don't use them all, but love the ones I use. Fun and flexible. Even techno-phobic teachers quickly warm to these activities.
Prezi - I haven't mastered this yet and my 'Prezis' aren't all I'd wish, but love the fresh angle on the presentation thing.
Webnode - my favourite free website builder. It doesn't get much of a press, but is a gem: clever, intuitive, flexible and free. What a great way to present a class project.
That was hard! So many great tools didn't quite make the cut. I must try this again in a few months.
