Heroshi, open source web crawler.

Motivation 1: learn HTTP, libraries, real world quirks.
Motivation 2: collection of libraries and tools for building custom crawlers.
Motivation 3: provide access to representative subset of Web for educational and research purposes.

As of 2012-10-12, last goal is not even started, but these guys did amazing job at it http://commoncrawl.org/

See http://temoto.github.com/heroshi/ for more information.