When (if ever) should links be opened in a new window?

I have seen a lot of discussion on this topic but it all seems to be opinion based without any research backing up the assumptions.

It seems that there are times that it is particularly relevant to open links in new windows (providing help or interrupting processes) but what about the more ambiguous actions (i.e. links to external sites)?

Does anyone know of any (preferably recent) research on the affect links opening new windows has on users?