There are 3 levels at GameGoo.  Explore the site to see where your child should begin. http://www.earobics.com/gamegoo/gooeylo.html

Primary Games has more than just reading and language arts.  It also has math, social studies, science, puzzles, and coloring activities.
http://www.primarygames.com

Storyline Online is brought to you by the Screen Actors Guild and contains professionally created storybooks delivered over the Internet.
http://www.storylineonline.net/

Seussville is a great, fun place to play and work with The Cat in the Hat! http://www.seussville.com/main.php?section=home&isbn=&catalogID=&eventID=

Letter recognition and number recognition site.  Also contains time activities for learning how to tell time.  Good for T-1 and 1st grade.
http://www.create-a-reader.com/Components/CARPage.swf?page=Letters&site=basic&lang=English

The BBC has many unique and excellent quality activities for all levels, including pre-school.  Pre-school children need few advertisements, if any, on the page.  Advertisements distract and children tend to click what ever looks interesting not knowing that it might be an advertisement.  The Starfall.com web site is the best I have seen with the fewest distractions.  The BBC site is also a good site. http://www.bbc.co.uk/schools/websites/preschool/

Even though there are advertisements and places pre-school students can click and move away from the site, there are a wide variety of pre-school activities at Funschool's Pre-school Arcade.
http://funschool.kaboose.com/preschool/games/index.html