Here are my favorites for Sammy's age (2-3ish) when he wants to play "compoo-ter" for 10 minutes. :)
Elmo's Keyboard-o-rama - They can touch any number or letter and a picture and the letter will come up and it will say, A, apple. Or for the numbers it will count up to the number of kisses, or flowers or balloons. Sammy has loved this site for a long time!
Kneebouncers.com - This one my mom just read about in Parenting Magazine (I think). Sammy likes this one (and Mommy too) because it doesn't really matter what key you hit things will happen on the screen (i.e. the train's whistle blowing, or the person playing peek-a-boo). They don't have to know how to manipulate the mouse or the keyboard to have fun.
Starfall.com - This one you need to be able to help them because you have to use the mouse but it is great for big kids too. It can help with learning letters all the way to learning how to read!
P.S. Besides the fact that my kids are still talking in their beds and not asleep for naps yet I'm loving my summer break! I've been off about 10 days so far and I keep thinking, oh, it's Saturday, (and yes, today it is) but the other 8 times it wasn't and it was great! I love feeling like every day is Saturday!