There are two free photo editing tools which are quite fun. Piknic is free and has lots of cool fonts and effects. Also, Sumo Paint is a relatively robust tool for drawing and photo editing. If you search for free alternatives to Photoshop others will likely show up in the results, however these two seem to have a robust enough. One caveat, you will likely want to create a white background photo for Piknic, because it needs a photo/picture to manipulate to work--as far as I know.
Of course there is always Gimp, which is an open source version of Photoshop.
Of course wikis and delicious are great for older kids if contextualized in an interesting and meaningful way.
I teach at the university level and have incorporated Google Docs into my course.
Students use a Gmail account to access an online, collaborative document--It has been most useful for exam reviews. I post a set of questions, and they collaboratively create the answer key. It take more effort on their part if there is a disagreement about the right answer, but it encourages them to question why an answer may or may not be correct (which is an important skill).
Many of them study in groups already--Google Docs helps coordinate this group studying in a virtual environment with more participants.
Another excellent learning tool that's becoming very popular is visual mapping (aka mind mapping).
Visual mapping is a whole brain system (combination of right and left brain) to capture all your important information. It’s a matter of taking information that you have and with which you are presented and then organizing it in a more useful and meaningful way. By using visual maps, you can better and more quickly capture, consolidate, organize, and analyze information and knowledge, and enhance your ability to understand subjects, solve problems, and learn.
Think of a visual map as a tree, where the various outlying branches — the subconcepts — all connect back to the trunk or central concept. The elements of a given visual map are arranged intuitively according to the importance of the concepts, with the goal of representing accurate and meaningful connections among them.
Visual mapping is already successfully used in business (many of top companies use it) and education (in many UK schools, visual mapping is now a core learning skill in the curriculum), and can be easily learned and used by all students.