I'm not really into playing games on the internet, but I love Guess My Number.
It's quick, simple and yet strangely fascinating.
You can dip in for one quick go when you need a thirty second brain-break, or you can stay there for ages getting into tactics and strategy.
My best result so far was guessing 47 in just 4 guesses.
See if you can beat that.
You have to try it. You'll be hooked.