OK, it's a stretch, but with a name like neoK12 I had to go there. This site is amazing for learners of all ages. Going beyond the games and puzzles most sites like this offer neoK12 has a selection of "interactives" and quizes. Many of these have prinatble versions in case you don't have access to the computers or the internet at work. If that's not enough, neoK12 has a nicely organized repository of videos that are safe for students. While many seem to be from YouTube they play in a format free of comments and related videos (one of my personal pet peeves). You can tag these videos as to which grade levels for which they are appropriate.
What I really like though is the mashup neoK12 offers allowing students to create school presentations. Students can select images from Flickr by category that are Creative Commons licensed, easily add them to slides, read about the topic on Wikipedia in an embedded article and then write about each slide on their own. The final product is either saved to their account and/or published to the web. Each photo gives proper attribution via Flickr. It's not Sliderocket with all the bells and whistles but it doesn't need to be. neoK12 offers young students the opportunity to begin creating content in a safe environment. Bravo!