I saw something on the bottom of the forum main page:
"LinkBacks Enabled by vBSEO 3.0.0 RC6"

What do you think about it?

After searching around, the results are:

Linkback
http://en.wikipedia.org/wiki/Linkback

vbSEO (not free)
http://www.vbseo.com/
http://www.crawlability.com/vbseo/

Is there free/open source for open source forum systems, i.e. phpBB?