Craigslist, however, you have to be careful or viewers will flag you. Most that use this as a form of free advertising expect to get deleted.
Many forums have advertising areas where you can promote things. People also discovered that Social networks are very powerfull free tool to promote things.
It depends on what you want to advertise. Craigslist.com is great for "Real" services, like Carpet Cleaning, Web Design (well that is kind over saturated), IT Consulting, Computer Support, Landscaping, Tile Work, Painting etc.. If you want to advertise Ringtones or affiliate programs etc.. I would say that myspace is a good place.
Hi, Just started a new service. The website is in my sig. I dont know if it will be the best, but we are trying hard. See this threat to join for FREE: http://forums.digitalpoint.com/showthread.php?p=7317810#poststop