[user name=name site=livejournal.com] with <> of course. Also works for site=archiveofourown.org site=tumblr.com site=twitter.com site=insanejournal.com :)
You can use it for any site at all that has a sitename/users/username structure. Any at all. If you find a site that you use a lot that you'd like to link to regularly that has a different structure, then that can be added to the list of supported sites, ask at Support if you want this done and they'll ask the relevent person to decide if it's a site they want to add.
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<user name="User or Community Name Goes Here" site="livejournal.com"> (i.e.
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Sam.
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In addition to what the others have already mentioned, there's also this post by