May 25, 2012 update
I registered at Alcumus so that I could try out the website. The problems there are tricky. You answer math questions one at a time. When you input your answer, the program will tell you if you are right or wrong. If your answer was incorrect, you may try again or give up. If you give up, or if you submit a second incorrect answer, the program will explain how to solve the problem.
I think that this would be a good website for kids who enjoy challenging math. It also keeps track of your success and progress throughout the site.
November 16, 2011
I just found out about a website called Alcumus. I haven't tried it out yet, but it comes highly recommended from a good source. The site says, "Alcumus offers students a customized learning experience, adjusting to student performance to deliver appropriate problems and lessons. Alcumus is specifically designed to provide high-performing students with a challenging curriculum appropriate to their abilities"
There are other math resources for kids who enjoy math on my student math page.If you find a resource that looks promising, please let me know. Maybe I'll post it to my site.