Fact: Outbound links on Wikipedia are nofollowed.
Implication: These links don’t pass “link juice” to the link recipient.
Result: When SEOs talk about getting links from Wikipedia, the main benefit that’s often mentioned is “it’s still a great source of traffic even if you don’t get the link juice.”
That’s true, but we’re overlooking one Big Fact: Wikipedia content doesn’t only appear on Wikipedia.com. It’s freely licensed, and liberally used by other sites.
And the external links aren’t always no-followed on those other sites.
Want an example? Here’s the “External Links” section of the Sears, Roebuck, and Company page on Wikipedia itself:
The links above are pink because they have the no-follow tag and I’m using Aaron’s SEO for Firefox extension.
Now, here’s the “External Links” section of the Sears, Roebuck, and Company page on Answers.com:
No pink. Cool, eh? That page on Answers.com is PR=4, which ain’t all that exciting … but you wouldn’t turn it down, would you? Of course not.
Now, just being happy you have a followed Wikipedia link isn’t enough. You have to help make sure that link holds some value. Here’s what you do:
1. Grab a bit of text from the page where your link is do a quoted search for it in Google. Here’s an example search with text from the Sears page. As you’ll see, most of the results are supplemental; if those pages have your link, I doubt there’s much benefit, if any.
2. So, if (when) you find a good result that’s not supplemental AND has the links followed, like the Answers.com example above, you need to help that page stay out of the supplemental. Give that page a link every now and then.
Conclusion: Links from Wikipedia probably are great for the traffic, but thanks to that content being licensed elsewhere, they can also be good for SEO reasons.
Needless to say, I’m not encouraging anyone to spam Wikipedia with links to your sites. No one with his/her head screwed on straight thinks that’s a good strategy.