I have MarsEdit. It did not seem to want to auto configure for LiveJournal like it does for my other blogging accounts on other services.
I spent over half an hour googling documentation, trying different settings, and seeing what happened. I finally hit on the right settings to use. They work, though there is a beep when I click OK. So, take this as good not perfect and enjoy.
Name: TITLE OF YOUR BLOG (free text, does not have to match exactly)
Home URL: http://yourblogname.livejournal.com/
RPC URL: http://www.livejournal.com/interface/blogger
Blog ID: YourBlogID
Number of posts to download: 10 (default)
If your Blog ID is JohnSmith - then enter JohnSmith in that field; make sure you use exactly the same case for each character as your Blog ID uses, just as if it was a password.
In the home URL, I am not sure, but I think those are always all lower case. Not even sure if case is an issue there.
The RPC URL field you should have precisely, character for character as I have it shown here. You might want to copy-paste it from here to that field in your options dialog just to be sure you don't get any typos. I copy pasted it from my dialog to here, so it is good, I promise. At least until they change the URI someday.
Here is the official how-to page for LiveJournal settings in MarsEdit:http://ranchero.com/netnewswire/tips/livejournalconfig.php
For another example, see settings in this bug report which seems to work now despite the problem the poster was having at the time:http://ranchero.com/bugs/display.php?bugid=2292
And here is support from MarsEdit. Read the posting by Brent Simmons at 05 Oct 2004 01:13:26 am :http://inessential.com/?comments=1&postid=2936
That should do it. Have fun with MarsEdit and LiveJournal now!