I found this free site which offers exercises which you can use to practice memory, concentration, and reasoning skills:
There are a variety of basic memory exercises, mainly testing visual/spatial immediate memory. Many of the tests are on a timer, for 90 seconds to 3 minutes, so the exercises are designed to help develop speed and accuracy. Unfortunately, there is not much at this site for practicing verbal memory skills, verbal comprehension, calculation, longer-term memory, or other practical cognitive skills such as remembering faces or names. But for a free site, it is quite good. It also shows you a graph of your score improvement over time, which can demonstrate to you that your skills are improving with practice.
The authors of the site are two British professors who do research into web-based assessment and development of cognitive skills.