All platforms with Firefox: If you like Delicious' ability to filter bookmarks by multiple tags (like "programming" and "tutorial"), you'll love the TagSifter extension for Firefox. Now that Firefox 3 supports bookmark tagging—and you've got keywords assigned to all your favourite URLs—TagSifter can help you navigate, search, and drill down to exactly the link you're looking for. Like Delicious, TagSifter adds related tag suggestions, and offers advanced search operators that can find exactly the bookmark you're looking for. For example, the expression:
muffins - (cookies + brownies) ?donuts
will show you all bookmarks (and related tags) that are tagged with muffins but not with either cookies or brownies and whose names or URLs contain the string "donuts".
TagSifter adds a sidebar for browsing your tagged bookmarks (shown right) as well as a drop-down menu from Firefox's toolbar (shown top). If you're not already using Firefox 3's tagging capabilities, TagSifter's import wizard will convert your current bookmark folders into tags for you. (The import wizard takes subfolders into account, too—so if you've filed Lifehacker under blogs > life hacks folders, TagSifter will tag the URL both "blogs" and "life hacks."
TagSifter is a free download for all platforms running Firefox 3.