I like to have an interviewee explain their previous projects and what they did. From this answer I can have followup questions: why they did things a certain way, how did they solve a particular problem if they mentioned one, but most importantly what was the purpose of the project and what business problem this solved.
I do this to see if they can articulate in a way that makes me understand what they were doing, and see if they understood what they were doing as well.
It's surprising that the last question about the purpose of the project and the business problem solved that trips a lot of people up. They have no idea WHY the project they were working on was being done at all. If you don't know why your project exists in the first place, it makes me wonder if you are contributing solutions, or just doing as you are told.
(Figured I throw this one in there, since all the other answers tend to be technical. I want people that know why they are solving the problems they are solving too, otherwise, they tend to solve the wrong problems that the end user doesn't care about :)