Here is my last weekend project that turned into useful extension. Basic version (Chrome Extension) will always be free and I will try to make money charging for SMS notifications.
Is craigslist officially okay with this kind of app? Prior attempts to automate, with varying degrees of aggregation/automation, have been shut down (see: http://www.wired.com/entertainment/theweb/magazine/17-09/ff_...). The "SMS coming soon for $3.99" seems like it would not fly.
I'm also working on a Chrome and Firefox extension which uses Craigslist as a data source, and this is my understanding as well
I think that rich client-side js apps packaged as browser extensions offer some interesting opportunities in general - being able to behave more like a normal client, instead of a giant aggregation host, allowing you to avoid angering services like Craigslist, being able to ask the user for permissions for increased localstorage limits (the default in most browsers is a measly 5mb) and the ability to do cross-domain ajax requests, etc.
If this thing does anything to de-dupe, it would be a must-have, would-pay-for-it app.
Trying to use Craigslist to find a new apartment means sifting through the same listings, posted day after day after day. They lower the signal/noise ratio a great deal.