If you can’t afford licences for concept mapping software, then you could consider introducing your students to Gliffy, or Bubbl.us. Both are on-line concept mapping applications. As with all the better Web 2.0 applications, you can use Gliffy and Bubbl.us to collaborate in constructing concept diagrams and other visual organisers.
You can only only draw 5 free maps with Gliffy before you have to pay though you can continue to modify the original five. The Gliffy diagrams are also not "live" in many blogs or wikis, you have to make do with an image capture as below.
The bubbl.us maps by comparison can be moved around within the blog or wiki as below.