Several innovative solutions have been invented over the years, the most obvious one is to do the CL scraping on the client, this way there is no indexing server to block, it's just the client reading all the search listings, parsing them and offering faceted filters. It breaks the spirit of the TOS but it would be impossible for CL to block. I am not advocating this, just pointing out one of the many approaches that have been attempted over the years.
Google can scan apartment listings and show them as results in their format. PadMapper cant. What's the difference?
As a limited exception, general purpose Internet search engines and noncommercial public archives will be entitled to access craigslist without individual written agreements executed with CL that specifically authorize an exception to this prohibition if, in all cases and individual instances: (a) they provide a direct hyperlink to the relevant craigslist website, service, forum or content; (b) they access craigslist from a stable IP address using an easily identifiable agent; and (c) they comply with CL's robots.txt file ...
All the other companies that have tried have also been told to C&D. If you don't like it, start your own network...
From http://www.craigslist.org/about/terms.of.use
"Any copying, aggregation, display, distribution, performance or derivative use of craigslist or any content posted on craigslist whether done directly or through intermediaries (including but not limited to by means of spiders, robots, crawlers, scrapers, framing, iframes or RSS feeds) is prohibited."