Taking the simple ones first, the following are all tools, which have the potential for making teachers' professional (and personal) lives a little simpler.
If you are a Twitter user, then you will be very aware that, as you follow more people, your timeline will become increasingly full of disparate tweets. Twitter Lists are a way of managing all these tweets, by grouping the people and organisations that you follow under headings and themes, e.g. tweets from local schools, tweets from those interested in educational technology, comedians, etc. The trouble is I haven't found a Twitter client yet that makes managing these lists an easy (or simple) task. TwitBird on iOS does a reasonable job, but is still quite convoluted. TwitlistManager looks promising, however. It give a spreadsheet-style view of all the 'tweeps' you are following and you can assign them to lists by checking boxes. Doesn't seem glitch-free just yet, but there is the potential there to make Twitter much simpler to use and more manageable.
Splashup is at the other end of the scale in terms of simplicity and is the first of my 'challenging' tools. It provides a fully featured image editor online should you want one, though I can see little point when desktop applications, such as Paint.NET or PhotoFiltre, are free and much easier to use. Aviary Image Editor would also fall into this category, though you might find it easier to use than I did.
When it comes to tools for teaching and learning, especially for young children, then simplicity and straightforwardness would appear to be a key attribute. The British Council's Learn English site exemplifies this very well. Although intended for learners of English as an Additional Language, it illustrates the point that good practice for EAL is good practice for all learners. Many of the games and activities on the site would be appropriate for developing literacy knowledge and skills for all children. I especially like Storymaker, which motivates writing (and reading) by creating a story from elements chosen by the user. Good stuff.