I came across the Luminosity page today. There are a few of these "brain-teaser" sites around, but this one seems more recreational. The concept; you play a number of games where you test and improve your skills in all kinds of mental activity; spatial orientation, verbal fluency, task switching, you name it.
I for one especially like the verbal fluency games, but I recognise that I probably also should work a bit on face-name recall. If you are really into this sort of thing, you might even want to subscribe to get access to some restricted material. I prefer to skim the top, however, due to my quality-for-free attitude to the web.