museumbox is pretty cool. http://museumbox.e2bn.org/index.php
http://www.edmodo.com/ Edmodo looks a lot like facebook but you can set it up just for your class.
I haven't found a site like it on my own but have you ever used www.siteslike.com? It allows you to search popular web destinations and gives you a list of similar sites to what you are searching. I went on there and searched "Glogster" and it returned 50 different sites that were similar. Try it out if you've never done so!
http://poster.4teachers.org/index.php. It is free and you can set-up one for your students as well.