I've been playing around with Symbaloo. I really like how easy it is to customize a Webmix around a concept. I also like that it is visually appealing. Finally, I love that I can share a Webmix with colleagues and students.
Here is a symbaloo playground I created around the concept of building digital playgrounds. It includes lins to some cool learning playgrounds as well as some links to resources about using digital content playgrounds in the classroom. Click the image to link to the Webmix.