Any good brain games?

[Bob]

Apps, whatever.  For a computer, online webpage, Android, or iOS.

Any good, free ones?  Emphasis on free?  Cause I'm cheap.

[quantum]

The one I like is blank staff paper and pencil.   

[perprocrastinate]

Chess is an excellent brain game.

[lmpianist]

Any of the Japanese-style logic games linked here:

https://www.puzzle-nurikabe.com/?size=5

I do a few of these just about every day after work.

[eldergeek]

Try Pure Mathematics: totally free, needs only pencil and paper, winner gets a Fields Medal and a warped view of of the universe for the rest of his/her days

[ted]

Try Pure Mathematics: totally free, needs only pencil and paper, winner gets a Fields Medal and a warped view of of the universe for the rest of his/her days

Yes, I agree with that one, best of the lot, especially in these days of the internet, where you can find people's theses, papers, lectures and the like free to download and study. Much remains to be discovered, even in quite simple things. Also, if you have a compiler of some sort, even a cheap one, heaps of explorations and discoveries are waiting.