We are selling our house for sale by owner right now, and are under contract.
The absolute best site that I found is zillow.com . It's super user-friendly, you can post as many photos as you'd like for free, you can very easily update it to note an open house, and we had plenty of calls/traffic at our open houses just from using it.
Things that we found to be key:
* clean the living fuck out of your house. Purge, empty closets (this may be the one time that I'd advocate for getting a storage unit short-term), clean baseboards, etc. Paint is probably the easiest way to really freshen up a house. Go above and beyond on cleaning, especially if you have pets.
* don't take crappy photos on your phone. Look at other houses in your price range on Zillow, and see the difference between nice and bright photos that some people post vs. crappy cell phone photos that other people post. The difference is night and day. If you don't have a good camera, ask a friend to come over and get some good shots.
* have an open house every single weekend. If you're selling it yourself, this is basically the only real marketing that you can do. Get your ass over there from 1-4pm every Sunday. We baked chocolate chip cookies and had water available, because our house has been sitting empty and has that empty smell. We made sure that all of the curtains were open, lights on, and bought nice new towels for both bathrooms.
* once you get your Zillow post up, post the link on Facebook and ask friends to share it. Re-post it about once a month. We got several calls from doing that.
* buy some cheap open house signs and put them on the main streets leading into your neighborhood. We had lots of people drop in because they saw the signs. (Ours were actually old garage sale signs that we re-purposed.)
* make a quicky flier to have in your house with a photo (people who go to many open houses need a reference for which one is yours), a detailed listing, school district, etc. We also noted the average utility bills for the past few years, as that's a real selling point in our old house.
That's about all I've got. Zillow, house/yard in ship shape, good photos, frequent open houses.