If you're trying to find a great deal, its hard to beat Craigslist. Although, I find it annoying that you can't setup e-mail alerts on the site for my favorite searches.
What I recently discover is that you can get RSS feeds from the site for any search. To subscribe to a Craigslist's RSS feeds:
- Go to your city's Craigslist site.
- Enter your search terms (keywords for what you're looking for), then press the search button
- Scroll to the bottom of the page, there's an orange button with the letters RSS, click on the button.
- Copy that page's URL and paste it into your favorite RSS feed reader.
If you don't like the suggestion above, search for "Craigslist email Alerts" to find companies that offer this type of service.