Zotero [0] is very good. it:
- is open source and well maintained.
- could be used both online and locally.
- offers both folders and tags for organization.
- could also be used for your pdf-annotation and generic web bookmarking needs.
While browsing sites like amazon and open library, you can capture the current page/book with all its metadata (author,year...) directly to Zotero in one click.
The heavy-weight champion remains Calibre [1]