As "baby boomer" rapidly becomes a nice way of saying "senior citizen," all sorts of games and brain-training exercises are hitting the market. Several of these products are reviewed in Putting Brain Exercises to the Test (Wall Street Journal, Feb 3, password required). Some have been through controlled studies, with results reported in peer-reviewed journals.
Posit claims it has solid evidence that their program works: Their web site earns high marks for transparently presenting the science behind what they do, and the research being done to support their claims. Peer-reviewed results. Findings from a randomized, controlled trial were reported last year in PNAS: "Significant improvements in assessments directly related to the training tasks and significant generalization of improvements to nonrelated standardized neuropsychological measures of memory (effect size of 0.25) were documented.... Memory enhancement appeared to be sustained after a 3-month no-contact follow-up period. ...This study demonstrates that intensive, plasticity-engaging training can result in an enhancement of cognitive function in normal mature adults."
Test yourself online. Posit has an online test to introduce people to their approach.