Bubbleshare's a safe bet
One of the things I look out for in an online tool is how robust it's going to be in terms of lifespan. Online services, especially free ones, need some kind of model to keep them afloat, keep them innovating and developing with me. If they don't have it I might as well go off and find another tool to invest time in learning how to use. Even more true when I'm building services or working with teachers who have no or little time to learn new tools and need something that they can stick with on the same terms for a long time.
Bubbleshare, one of my favourite tools that was bootstrapping, it seemed, for a long time, has found its lifespan in a $2.25 million investment (it could stretch to $3m). We'd already started training teachers in East Lothian on how to use it for teaching and learning but we can do so with a bit more of a long term relationship now.