So Google does not allow people to buy links or exchange a link for a link. They also penalize for low-quality / spammy link building. Those are the two things that they don't like.
Here at Moz, we gamify our Q&A and blog comments with MozPoints. At 200 MozPoints, you get the first link in your profile followed. This works for us because we also heavily moderate in order to make sure people aren't gaming the system just for MozPoints. (Happens all the time.) Also, if you write a guest blog post for us, your links will be followed, but again, it's okay because we edit and make sure the links are relevant and moderate, moderate, moderate. Additionally, we have so many links pointing to us from a variety of sources and a ton of relevant links in both directions that a handful of potentially off-topic links are a drop in the bucket.