Don't let your web site fall out of the window
The question of whether to open a new browser window for offsite links came up on CHI-WEB. Lois Wakeman has a nice article about why this generally presents a problem for users. She includes some good examples and usage scenarios (see the sidebar). Personally, I've seen every one of the scenarios she lists occur with users in usability testing. I've seen users with 6 or more windows open (unbeknownst to them) as well as users who accidentally close their only browser window thinking it was another popped-up window. She also covers the topic of popup (or pop-up) windows with a nice working example.
I'm still looking for some guidelines that talk about good uses of popups. If you know of any, please drop me an email and I'll post a link here. I think areas where popups MIGHT be useful are user assistance (e.g. help, glossary, etc.) and showing full-size images when browsing thumbnails.
Generally I avoid designs that open new windows for users. I've yet to find a web user who couldn't find the Back button or some other way back to the previous site when needed.