There are several math enrichment sites I have explored.

IXL.com requires a subscription and covers all math skills by grade.  It appears to be useful for preparing for standardized testing format math.

Xtramath.org  Is a good FREE site for increasing speed on math facts.

Mathscore.com is a subscription based service and is incorporates speed and skill practice.  I really like that they teach the kids to us a 10-key correctly before anything else. Mathscore offers a free trial period.