a Dual N-back game based on the research presented in the Proceedings of the National Academy of Sciences. Studies show that training with Dual N-back tasks help develop general problem-solving skills; training for 20 minutes a day for 19 days, study participants were able to achieve gains of more than 40% in fluid intelligence in tests.
Read more on these studies at the links below:
Wired Article: Forget Brain Age: Researchers Develop Software That Makes You Smarter
New Scientist: Simple brain exercise can boost IQ!
Original Study: Improving fluid intelligence with training on working memory.