I think using a varirety of Mashups is very effective in the classroom. I really have never heard the word "mashup" before, but I realize that I have been using them for a long time. For example, I have been using Google Earth with my students when we Skype with other classrooms. I also am starting to use Google lit trips, which are awesome! You can literally take your kids are on a virtual trip while you are reading to them. My students enjoy Webowords, which is a really cool visual vocabulary site that offers illustrations and a little cartoon description of a vocabulary word daily. Students can also go back and review previous vocabulary words. I like to put the word of the day on my Promethean Board for my kids to see each morning when they walk in.
Another mashup my kids enjoy is Shahi, which is a visual dictionary to get students to look up words. It presents several visual representations of the word using Flickr. One more of my students' favorite mashup is Scribble Map. This is where they can zoom in to an area and type in the text box, draw, doodle etc. They can take notes on the maps or make a timeline... They can then email it to themselves. It is a great resource. I think it is pretty awesome how they are combining data sources to make something new and better.