After Google declared that sites selling text links without the nofollow tag would be penalized, this monetization method became less popular.
Many website owners are still using text links to monetize their sites, though, some using the nofollow tag and some not.
The advantage of this method is that it is not intrusive. One can sell text links directly through his website or use specialized networks like Text-Link-Ads and Text-Link-Brokers to automate the process.
Text link marketplaces and networks: