Blogs are one.... lots of places to blog.... Trulia, your company blog, Realtor.com, ActiveRain, your own blog. Blog not what you want to talk about, but what customers want to know about real estate. Establish yourself as the expert in different areas. Not only this, but you want a way to capture customer information when they get there.
We're having some training about this in our office tomorrow (Tuesday) about some of the free lead sources. If you would like to come let me know. 9am-12noon. This is with one of our master trainers.
One of our trainers also teaches at MetroTex about free lead sources and I highly recommend the class.
You might also google...free real estate lead sources and see what comes up.