How do I make a link to another Dreamwidth account, or an account on another service?

You can use the <user name=username> tag to create a link to another Dreamwidth account. The link you create will look different for a person, a community, or a feed, but in each case the icon will be a link to that account's profile. For example:

DW user: <user name=username> gives username
DW community: <user name=omnomnom> gives omnomnom
Feed account: <user name=xkcd_feed> gives xkcd_feed
OpenID account: <user name=rahaeli.livejournal.com> gives rahaeli.livejournal.com

You can also specify an account on a different site by adding "site=sitename" in the tag, as follows:

<user name=news site=livejournal.com> gives news
<user name=announcements site=insanejournal.com> gives announcements

This feature will work with any site that has specifically been added, or with any site that uses the structure "www.example.com/users/username" for their URLs. Unknown sites will display with a generic icon.

Sites that are currently recognized by this tag are:

archiveofourown.org (also works with ao3.org)
blogspot.com
deadjournal.com
delicious.com
deviantart.com
etsy.com
fanfiction.net
inksome.com
insanejournal.com
journalfen.com
last.fm
livejournal.com
pinboard.in
plurk.com
ravelry.com
twitter.com
tumblr.com
wordpress.com

To request that a site be added, you can make a suggestion.

Official communities and Dreamwidth staff accounts display with different icons, but the method of linking is the same. These official accounts display with different icons to show that they are in some way connected to the process of running Dreamwidth. For example:

DW staff: <user name=denise> gives denise
Official community: <user name=dw_news> gives dw_news

If you're using the Rich Text Editor, enter the account name, then highlight it and select Account. This will automatically generate a link. At the moment this only works for links to users on Dreamwidth.