Check out Scrapy:
http://scrapy.org/
It's perfect for large-scale scraping tasks. We use it for all sorts of one-time scraping tasks on my startup, http://parse.ly. Usually takes about 1 hour to write a scraper for a big site, and then the crawls run pretty quickly due to use of Python Twisted (evented IO framework). Plus, comes with a nice web-based console for monitoring crawl jobs in process.