It is possible to add web parts to default pages that show document libraries and lists. For example, when you are in a SharePoint site, and you go to Documents and Lists and you then click a list or document library, you can add web parts to that page.

Just put the following line in the address bar of Internet Explorer followed by <ENTER>:

javascript:MSOTlPn_ShowToolPane('2');

This opens the toolpane to add web parts to the page. Replace two by 5 to open the import web part tool pane.

More info here: Open List Items in a new window. In the article, Todd Bleeker explains how to use this technique to add a content editor web part with extra javascript to open documents in a new window. Amazing stuff really!