This is a list that I created in response to a request that I recently received as direct message on Twitter. A couple of these resources I found through a search in the Diigo community. If you don't have a Diigo account or if you do but you have never searched the community bookmarks, I recommend giving it a try.
Who Wants Pizza? is a fun online activity for learning about fractions. Who Wants Pizza was developed by Cynthia Lanius at Rice University. The activity has five parts plus practice activities for students to explore. Teachers will find notes about using this activities in the classroom.
Visual Fractions has eight categories of visualizations, lessons, and games for students to explore and learn the functions of fractions.
The National Library of Virtual Manipulatives has a lot of interactive activities for students learning the use of fractions, addition and subtraction, and the multiplication of fractions.
Thinking Blocks is a nice site for elementary and middle school mathematics teachers. Thinking Blocks provides interactive templates in which students use brightly colored blocks to model and solve problems. As students work through the problems they are provided with feedback as to whether or not they are using the correct sequence to solve each problem. There are templates and problems for addition, multiplication, fractions, and ratios. You can also develop your own problems using the modeling tool.
Conceptua Math is a provider of interactive visual mathematics lessons. One of Conceptua Math's primary focuses is on the development of tools to aid teachers in the instruction of lessons on fractions. Conceptua Math's offerings are a mix of free and premium (paid) tools. There are a total of fifteen free interactive tools for teachers and students. Each of the free tools has an introductory video and a sample lesson plan.