I recently discovered padmapper while apt hunting, and it's a godsend. It scrapes craigslist for data and posts it to a map.
The killer feature is obviously to be able to look at a region and see what's available, but the filtering options are also way easier to use than craigslist's rather pathetic interface.
This is funny because housingmaps, mapping craigslist listings to google maps was the original mashup. The guy went and worked for google because he said he didn't think there were any good barriers to entry to run the idea as a business.
