But then his data source is gone and what he was doing is pointless. Losing your processor of said data source while said data source is still available is frustrating.

Scrapy gets a solid recommendation from me. http://scrapy.org/

I use custom node.js scripts with these libraries:

* request - https://github.com/mikeal/request

* async - https://github.com/caolan/async

* cheerio - https://github.com/cheeriojs/cheerio

* nedb - https://github.com/louischatriot/nedb

We've got quite an old mailing list full of geeks hand-coding web scrapers, if you want somewhere to ask questions:

https://groups.google.com/forum/#!forum/scraperwiki

I've written a couple "polite" crawlers in Go (i.e. obeys robots.txt, delays between requests to the same host).

- Fetchbot: https://github.com/PuerkitoBio/fetchbot

Flexible, similar API to net/http (uses a Handler interface with a simple mux provided, supports middleware, etc.)

- gocrawl: https://github.com/PuerkitoBio/gocrawl

Higher-level, more framework than library.

Coupled with goquery (https://github.com/PuerkitoBio/goquery ) to scrape the dom (well, the net/html nodes), this makes custom scrapers trivial to write.

(sorry for the self-promoting comment, but this is quite on topic)

edit: polite crawlers, not scrapers.