Craigs list seem to draw the most responses. Point 2 syndicates your listings to other sites.
You can try backpage.com
Realtor.com is good but you pay for it if you want more thaan just a one photo listing.
We use Craig's list as well. And we have another Canadian website we post on. I don't know of any others right now. Craig's list seems to be the most popular.
I try to get links to my site from real estate web sites in my state, such as the Maine real estate network or any site that advertises Maine real estate. I just google Maine real estate to find the sites and then request a link.
I also use MilitaryByOwner.com, since I'm in a military base-rich environment. Costs $89 for a featured ad with lots of pictures, but I've gotten several good buyer leads from it.
If you google "free real estate classifieds" the list is endless! There are a TON of free places to place those kind of classified ads for your lisitings.
Not sure what type of traffic or buyers you will get from it, but hey....it's FREE!
:)
Zillow.com works great for me.
Try postlets.com - it will then send it to at least 6 other sites and it looks great on craigslist