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When the link points to a file (for example, a PDF), Interclue shows its file size and the last modified date, as reported by the web server. This is useful if you don't have a fast Internet connection and you don't want to wait for an hour to download Adam Bosworth's speech about health. Fortunately, the file is quite small.
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The previews are pretty clever: for example, the preview for a link to a YouTube video shows the actual video. The extension "tries to avoid doing any look-ahead operations on [links that produce side-effects] by avoiding links with certain keywords in their text and link (such as 'logout', 'delete', 'remove' and so on), and also by turning off Linkclues for internal links on secure sites, which are typically used for banking, ordering goods and services".
You'll find a lot of things you can change in the settings and also the options to disable the previews for certain domains. Overall, the extension takes a great concept from Cooliris and transforms into a brilliant tool.