Brain Workshop is a free open-source version of the Dual N-Back mental exercise.

What if a simple mental exercise could improve your memory and intelligence?

A recent study published in PNAS, an important scientific journal, shows that a particular memory task called Dual N-Back actually improves working memory (short term memory) and fluid intelligence. This finding is important because fluid intelligence was previously thought to be unchangeable. The game involves remembering a sequence of spoken letters and a sequence of positions of a square at the same time.

Anecdotal evidence suggests that the dual n-back task also enhances focus and attention and may help improve the symptoms of ADHD/ADD.