I love to ask something that is simple to start with, and then build on it interactively with the candidate.

One of my favorite questions is - Give me a normalized database structure that you'll implement if you were to build gmail - incorporate conversations, messages, multiple message participants and labels.

Then, depending on the candidate, I build upon the question,and go into various optimizations possible, the ways caching would be implemented, sharding/splitting/de-normalization would be done with load, etc. etc. With good candidates, its always a very interesting discussion. And even the bad ones can leave the interview thinking they know something and don't need to feel bad :)