Stephen Hackett had a nice post up on his site about how and when to attribute content or links to other people.
Proper attribution is important and sometimes not that easy.
Yesterday he posted a simple attribution rule that Chris Martucci lives by:
My criteria for via links are simple: If someone points me to an article I did not find on my own (either through my RSS reader or at its original source), I’ll include a via. Likewise, if an article I found on my own links to a second article, which I then post to my weblog, I’ll link to the original source.
The only thing I do differently, is to also mention the site I found a link on by my own, if the author of the second article added even the slightest bit of value to the original post.