Link swapping and reciprocal link deals. Basically - if you give me a link on your site I'll give you a link on mine.
Simply put link swapping does NOT work and hasn't worked for years. If Google see site link to site B and site B link to site A it simply devalues that link to next to no value. Google see this as technically buying a vote or link by offering a link in return. Anyone who says this works is imply wrong.