This website activity asks students to find 10 hidden dog bones on a 100 chart in one minute. The trick is the chart is blank! A number is presented, such as 62. Students have to use their knowledge of the 100 chart, such as each row is worth 10, to hop around and pinpoint the number. This game would work well on an IWB or on a computer as a math center.
Another great website, this time from the UK. This activity is perfect for developing understanding of big numbers and place value. You are given a choice if you want to play with 10's and 1's, called "units" or 100's, 10's and 1's. This is a great visual tool! Again, this would be wonderful on an IWB or an individual computer station.
This website game, from Carstens Studio, needs Adobe Shockwave to play. I love this game, which is excellent for students who need a bit of a challenge with addition facts. Sums can be represented with dice, coins, or digits. Students click and drag values from three stacks, moving them around until they make the target sum at the bottom of the stack. There is a matching iPad/iPod/iPhone app available for $0.99, which has a few more options, including fingers and Spanish number words.
Hope you find one of these useful in your teaching!