Barring something useful like RSS feeds, you're going to have to do this on a site by site basis. What should ideally happen is your spider, when visiting a site, should load the rules for parsing that site. Maybe subclasses that override a &content method would be appropriate.

Regrettably, I do a lot of work like this and it's easier said than done. One thing which can help is looking for "printer friendly" links. Those often lead to a page that strips a lot of the extraneous information off.