i only recently noticed this, because i don't use the web client often. i post with a different account even less often, for that matter.
(or http://www.livejournal.com/update.bml?altlogin=1&mode=full for the full update page)
this takes you to the journal update page, with the option of making an entry with a different username, without having to log out of your usual account.
in the same vein:
that allows you to make modifications to the colours, overrides, and other style options of an account other than your default, without having to log out. extremely useful if you ever need to test some overrides or a colour scheme before using it on your own journal (or before you give the code out in support answers, which is what i use it most often for!)