I've always been a fan of: "Pick the project you have most enjoyed working on and describe in general terms it's architecture." Allow them to use a whiteboard and listen carefully to the terms they use. How well do they express themselves? What level of detail do they go to without prompting? Do they use general pattern nomenclature? Towards the end start challenging technology decisions and see how they defend them. Ask them how they tested the software. Ask them what they'd change if they were able to start again.
Also, I've never asked this one but: "Image that everyone who has been maintaining your code for the last 5 years is here right now. What do you say to them?"
