Choose your starting number. Then click repeatedly on only those ghosts which are multiples of that number. How long does it take to reach 100?
Three thumbs up!

http://www.oswego.org/ocsd-web/games/Ghostblasters1/gbcd.html

Prime Factors Calculator
Type in a number (it can be a large number but not TOO large) and click {OK} to see its prime factors.
http://newmanservices.com/primefact/

The Nth Prime Number
Enter a value for n below, from 1 to 10^12, inclusive. The server will return the nth prime number (counting 2 as the first). Commas and scientific notation (e.g. 1.0e12) are allowed.
http://primes.utm.edu/nthprime/index.php#nth

Factorise
Visualize factors through building rectangular areas on a grid. First enter all the factorizations of a number, then draw each factor set as an area on the grid. Factorize is one of the Interactivate assessment explorers.

http://www.shodor.org/interactivate/activities/Factorize/

Factorise II
Visualize factors through building rectangular areas on a grid. As you draw each factor set on the grid, the factors will be listed.

http://www.shodor.org/interactivate/activities/FactorizeTwo/

An interactive 100 square. You can choose to start the square with either 0 or 1.

http://www.hellam.net/maths2000/100square.html

You can also change the 'width' of the 100 board using this link:

http://www.hellam.net/maths2000/nsquare.html

Click on two cards to turn them over. If they match, they are cleared from the board. If not, they remain on the board. How many turns will it take you to clear the board? But there is a twist - you also have to click on the 'odd number out'!

http://www.interactivestuff.org/match/maker.phtml?featured=1&id=17

A highly interactive Promethean ActivBoard flipchart where students discover the prime numbers, factors of numbers and the Highest Common Factor using a game based on Eratosthenes Sieve. Three thumbs up!

The flipchart can be found here:
http://www.prometheanplanet.com/uk/server/show/ConResource.13015

If you are not yet a member of Promethean Planet, you will first have to register here:
http://www.prometheanplanet.com/uk/server/show/nav.914

If you want to view the activities and have some interactivity with them, but you don't have the ActivStudio software, you can download the free Viewer from here:
http://www.mypromethean.com/uk/server/show/ConWebDoc.2974

Join together cogs of different sizes and investigate what happens when you turn them. From the NRich website.
http://nrich.maths.org/public/viewer.php?obj_id=4771&part=note

Starting with a 100 Grid, the first player chooses a positive even number that is less than 50, and crosses it out on the grid. The second player chooses a number to cross out. The number must be a factor or multiple of the first number.

Players continue to take it in turns to cross out numbers, at each stage choosing a number that is a factor or multiple of the number just crossed out by the other player. The first person who is unable to cross out a number loses.

http://nrich.maths.org/public/viewer.php?obj_id=5468

Students practice their knowledge of factors and products. There are links to lessons on how you might use this engaging game in the classroom.
http://illuminations.nctm.org/ActivityDetail.aspx?ID=29

The factor game is a classic! Play against a friend or play against the computer. All you need to win is a good knowledge of factors and a bit of logic to figure out a winning stragegy!
http://illuminations.nctm.org/ActivityDetail.aspx?ID=12