You could request the candidate to retrieve an assignment from a password-protected website and require a solution within X hours after retrieval.

This does put on some pressure - there is a deadline, after all - but people who are shy but confident in their coding abilities may be less anxious. This would also solve the common "but I wouldn't code without an IDE/Google/Wikipedia" complaint.

Having experienced it myself, I can vouch that such an difference exist. When forced to write a complicated piece of code with a bunch of strange people breathing down my neck and watching every step I take, my productivity tends to slacken off when I hit a coders equivalent of writer’s block. It just doesn’t come out as naturally as it used to. In contrast, same task in a more familiar and comfortable setting looks like a piece of cake even if deadline is drawing close.