It turns out you can delete all the text in the "body" field and just write in whatever you want. Address the sucker and press "send." The result is an email to anyone on the net with your username @livejournal.com in the "from" field.
It's quite nifty if you need to send an note to someone, but school/work/library regulations forbid the whole Hotmail/Yahoo/etc. thing.
Now, I have a paid journal, so replies to such emails do get back to me. I'm not sure what happens if you try this with a free account, however.
One more thing. Like many undocumented features, the reason you didn't know about this is that it's probably capable of really straining the system. So don't abuse it, k?