Google Notebook bookmarklet is a lightweight alternative that doesn't require installation. You just drag and drop a link to your browser's links bar or bookmark the link. To copy some content from a web page to a notebook, just select the text and click on "Note this".
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For some reason, Opera can only view notebooks, so this bookmarklet can't be used to add notes.
Related:
Useful Google bookmarklets
{ via Google Notebook Blog }