I have a web-based bookmark site that I've created for my students to use. It contains many categories that we will be studying, as well as general interest sites for third graders! These are all websites that I've found and tried, and I do add to it and keep it updated throughout the year. It's called "Portaportal".
To access it, just type in www.portaportal.com. On the home screen, go to where it says "guest access", and type in jbabilino. You'll see lots of fun, safe categories of websites for kids to explore and enjoy!