Evernote is a note-taking application that allows you to maintain a number of “notebooks” that you can piece together from any source (it acts as a very intuitive scrapbook with indexing and OCR features for extracting text from images).
It has now morphed into a web service of sorts with mobile and cross-platform syncing support, which makes it extremely interesting (more than, say, Yojimbo and other scrapbook-like apps).