This is another tool included on here which is not free but we think is worthy of inclusion and of your consideration.  Realsmart offers a great deal and was build around the idea of assessment for learning.

Allow your students to build online eportfolios in a very visually and intuitive way.

Provide success criteria and learning outcomes in a very visual way so that students can measure their own progress, they can upload evidence of meeting that criteria and can also recieve feedback.

Here is a link to the Realsmart website.  Click on the third video (Realsmart: your learning portfolio) for an overview of what it can offer in terms of assessment for learning

A very simple idea which does not need much explaining but an obvious way to use technology to support assessment for learning is by ensuring that students know what it is they are to be judged upon.

Provide links to success criteria online as well as a range of exemplars for students to look at.  This way they know what it is they are aiming for and have something to self assess their work and progress against.