Ever since games such as Brain Age and Brain Trainer have become available, there's been a renewed interest in improving the brain.

I mentioned lumosity earlier this month, where you can improve your brain over 30 sessions.

In a more competitive vein, there's BrainSpeeder (also available in German). Once you register for free, there are numerous leagues in which you can pit yourself against people in arithmetic, mnemonic and speedoku competitions. You can even compete to be Brain Of The Day.

Over at BrainBall, you can play all of their games solo without registering. If you want to pit yourself against others, you will have to register.

However, there are two completely registration-free alternatives. First, for those who haven't checked it out already, I'd like to shamelessly plug the Grey Matters Mental Gym. Here, you can learn a wide variety of memory techniques, learn some amazing memory and mental math feats and even practice them!

There are also some obviously kindred souls over at Brain Metrix. There are some excellent articles and activities here. If you enjoy this site, you'll also enjoy Brain Metrix. This is one of those sites you can easily spend several hours exploring.