TrackStar has been around now for ages, but it is still a site worth mentioning. This is a terrific, free website that "bookmarks" sites that you approve and pick out from around the web. It's a great resource, because you can either create your own tracks or you can browse premade ones from other users. You are also able to annotate the different sites you set up.
You can find TrackStar at http://trackstar.4teachers.org/trackstar/index.jsp.
I use this site to handpick websites that I think will be beneficial for my students and that match our curriculum and standards. I create math and reading tracks, and I put a bookmark for each track on the laptop desktops for my students to use during centers (I usually go in and change the icon to a "star" also so they can access it and see it easily). This ensures that the students are only going to the websites I've approved of and not just randomly searching the web (this still takes training though with young students). I also sometimes send these tracks to the parents so their children can use the same sites at home for consistently. Enjoy exploring this wonderful resource!