How does everyone feel about the concept of "exchanging marketing assets"? This would mean for example someone giving you a guest post on their website in exchange for a one month banner advert on your site, a face book like for a re-tweet, etc. The beauty is that no money changes hands and what you see as a simple and cheap marketing asset in your portfolio (such as a website) could be worth more to someone else.
Like4like are still prevalent. You can search the FB groups relevant to your niche and contact the owner with your offer. Generally if you have equivalent likes in your group (give or take 10%) people are ready to exchange. However, the example you gave...guest post for banner advert...I guess people would not like to barter there. You can contract in terms of money with them though.
Hypism.com is something I started to "exchange" popularity. Let me know what you think, feedback button on the page.