Who do you put on your blogroll?

As a newbie attempting to keep up on a blog, the blogroll seems to be a pretty basic part of one’s site.  How do you decide who to put on there?  Any specific criteria?

I’m also wondering when you allow others to actually post entries on your blog?  Is it more accepted to have a site dedicated to multiple authors, or to sporadically let one or two others post on your own blog?

Let’s see where this one goes…