craigshunter.com
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.
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.