www.miniclip.com is fantastic. Has loads of sections of games - skill,verbal,speed etc. About 100 games easily I should think, some harder than others. You can try them all, and the ones you like you can buy and download them.
the nickolodean(?sp) website.
my ds1 who is just 8 would spend all day on it if i let him and no pop ups that you wouldn't want them to see, unlike another website that he used to use until i twigged where all the viruses and rude pop ups were coming from. I can't remember what it was called but will ask dh so that you can avoid it.