Eigenpoll : Cognitive improvement : N-Back computer game 

N-Back computer games are used to improve and measure working memory.

If you consistently play it you will slowly but surely build up you working memory, meaning you can hold more mental objects in you mind, this improve you thinking and you are able to solve problem that was to hard for you before the training.

You can also use it as a measuring tool, to see how well other tool improve you mental capacities.

To find some games just Google 'N-back games'

Here is a link to the Dual-N-Back FAQ