Just discovered a great riddle/puzzle site: [wu:riddles].
As the site says:
The riddles are organized by difficulty … easy, medium, and hard. Then there is the microsoft section, consisting of weird, open-ended consulting-style questions. The cs and putnam sections contain problems which may require a particularly specialized background in computer science and mathematics
Browsing around in the easy section (that was tricky enough for now) I came across some really nice ones. Check out Glass Half Full and Infinite Quarter Sequence.
I found the the glass half full and other riddles somewhere else, but I don’t like this particular riddle because the condition that it is a right cylinder makes it kind of obvious.