As I have worked with teachers, these websites have proven useful:

Math         Language        Science        Social Studies
          Primary Resources        Thinking Tools        Collaborative Tools        Images

Websites that support education are limitless.  Locating the sites that meet your classroom or student needs can be time-consuming.
Using refined searching with google can provide sites to meet your needs, but try some of these resources as well.

Social Bookmarks

Using a social bookmarking tool such as can be helpful when searching out sites that support a particular topic since these sites use keyword tags and can be searched using those tags.  The tags are words that the person who added the website to delicious felt described the content on the site.  (An explanation of social bookmarking:
Here is an example of a search for math websites for interactive elementary geometry games:
For other subjects/topics think of words that describe what you want to find and enter them into the search on delicious.  You will get a representation of websites on that topic that other people have liked well enough to add to their delicious accounts.

Another bookmarking site is Diigo and you can perform similar searches there as well.

Having an account on a social bookmarking site allows you to personalize sites by adding them to your delicious account with tag words that make sense to you.  These can be easily shared with students by having them visit the url web address created by a combination of your account and the tag word. 

Here is an example of the url for sites I have marked as spelling in delicious:

Excellent Resources
This website consistently adds links to education resources and includs reviews of the links to help determine if they will enhance learning for your students:

The Tech Chicks from Texas have a somewhat regular podcast and share educational links accompanied by reviews of the links.