You might already know that on pages like http://www.livejournal.com/update.bml and http://www.livejournal.com/modify.bml there's a link - "You are currently logged in as [username]. If this is not you, click here." The wording of this link is somewhat misleading, because it does not actually log you out - it allows you to update or modify some other journal while staying logged in as whatever you're logged in as.
All these links add ?altlogin=1 to the end of the URL - for example: http://www.livejournal.com/update.bml?altlogin=1 displays the update page with a username and password prompt.
However, http://www.livejournal.com/editinfo.bml does not seem to have this. It has a drop-down box where you can select "Other" to bring up a username/password prompt, but the URL still appears as http://www.livejournal.com/editinfo.bml - for the geeks among you, this is because the form produces a POST request.
In reality, editinfo.bml does have this, although it's not "?altlogin=1". The following URL does it:
If you're using several journals and frequently want to update their bios, interests, or other information, bookmarking this URL may be helpful.
Update: Please notice that this is the URL that displays a username/password prompt. You do not replace anything in this URL. The percent signs (%) are part of the URL.
Update 2: This is useful if you use several personal journals or shared journals. If you "only" maintain communities, then you can bookmark the following URL directly:
where, of course, you replace "communityname" with the name of your community. This way you don't even need to enter the community's password, but it only works if you're logged in as the maintainer.
Update 3: (sorry about all these updates...) If you go to the following URL, you will be able to edit your own personal info. This, of course, is useless because you can already do that by just going to editinfo.bml. ;)