Evernote is perfect for using as a commonplace book [1] for exactly this purpose. The only drawback is that it's hard to view in a chronological order, which is a key part of the commonplacing idea. Seems like a blog platform on top of it could solve this right away.

Now I just need to get around to reading those books I want to excerpt from...

[1] http://en.wikipedia.org/wiki/Commonplacing