Always check out if the site that you are linking to is "legit". That means, it is not banned by google. Which further means that it needs to be indexed and preferably have some PR (page rank).
There are tools that can do this job for you, but unfortunately I can't remember what to type to find them. Try performing a search on "bad neighborhood check" or something like that.
Basically, a nofollow link will not pass any "link juice" (page rank), but it's still considered valuable as it can drive traffic and
maybe improve your search result positions.
To back this up, I remember some SEOs doing tests and some of them figured that if the no-follow link comes from a good relevant site, it can possibly slightly improve your rankings. Nobody knows for sure, only google's employees.
As far as guest posts are concerned, people usually want a "do-follow" link so they can get the most out of it. This means that your outgoing links cannot look like this:
<a href="http://www.website.com/
rel="nofollow">link</a>
You also can't have:
<meta name="robots" content="nofollow" /> in your meta tags.
If the site that you would link to is "legit", there's no reason not to use a standard link (if you're ok with giving out some of your hard earned "link juice").
But even that shouldn't concern you since linking to "legit" sites (relevant as well) is a good thing for your site. Some people consider this also a ranking factor since you're contributing to the "user's experience" by providing more trusted quality relevant information. And, if you're getting quality content in return, everybody's happy. Just make sure you don't post any plagiarized content.
Use copyscape or take small samples of the text and perform a search in google: "a sentence or two from the text" (yes, use the quotes).
I hope everything is a bit clearer to you now. Here's an article straight from google:
http://www.google.com/support/webmasters/bin/answer.py?answer=96569
But remember, it doesn't necessarily mean that everything is exactly as it is written. I personally would always pick up a link from a relevant no-follow site (if there's really no chance to get a dofollow) because I'm seeing a lot of them in my webmaster tools account, and I have a hunch that they really do help a bit as far as rankings are concerned. But that's me...