That's exactly what List-Alert.com does. We send you an email when someone posts the thing you're looking for on Craigslist.
How does it work you might ask?
Craigslist provides an RSS feed that lists the titles and descriptions of the latest posts for a given search. Our service is an RSS feed reader that checks this RSS feed related to your search to see if the things you're looking for have been posted. When we get a hit, we send you an email to let you know.
When you see the listings on List-Alert, what you're looking at are the real-time results from the Craigslist RSS feed, and when you click on a post title, you're taken directly to Craigslist to view the post.
We also always provide a direct link to the Craigslist RSS feed in case you'd like to read it in your favorite RSS reader. In addition, we provide a link to the full search results on the Craigslist site so you can see all the posts (not just the latest ones).
The whole idea is to make it easier to find what you're looking for on Craigslist and avoid missing anything.
Anyway, I hope you find it as useful as we do.
Thanks for giving it a try,
Ted (programmer, creator) and John (business guy extraordinaire, co-creator)