I don't know about Macs, but for people with PCs there is:
- LJ Archive, which creates a searchable database of all your posts as well as all of the comments. Whenever you launch it, it automatically harmonizes the database with your online journal and all comments that have been made therein. It's a wonderfully nifty tool, in large part because of the searchable aspect.
- LJBook will back up your entire LJ to a PDF file. I've never used this one (although I probably should), but I'm sure other people could speak to its usefulness.
- LJ also has a a program that allows you to download and back-up. The more computer-savvy on my FList indicate that despite what LJ says, it is possible to take this file and re-upload it to WordPress. (Someone honestly needs to write a step-by-step guide to explain how to do this.)
In addition, you can back up your LJ to any LJ clone using LJ-Sec. I've used it to back up this LJ to InsaneJournal, JournalFen, and the on-its-last-legs GreatestJournal.
To be honest, I pretty much use IJ as my online back-up, but I haven't been terribly active over there despite the fact I have a perm account. JournalFen I pretty much lurk there, but only have a partial back-up of my LJ. I deleted by GreatestJouranl account sometime ago. (I'm under the same username on LJ, IJ, and JF.)
I probably should
Plus, I keep hearing information about something called DreamWidth, which appears to be a newfangled LJ clone but with some additional features(?). I don't know much about it one way or the other.
Anyway, if you want to back-up to an LJ clone using LJ-Sec, brown_betty has instructions how to do it here. You can read about my experiences using LJ-Sec as a backup here.
Hope this has helped!
ETA: Only because I had to go into a meeting and just got back. Personally, I'm not all that worried about LJ disappearing off the map. I had a feeling that there would be layoffs in San Francisco after our Russian Corporate Masters moved the server farm out to Montana.
I feel bad for the employees that got shafted out of severance and notice. They've got it sucktastic right now, and I keep my fingers crossed that despite the economy they'll come out of this okay.
I'm far less worried about LJ disappearing without a word or without a trace a la JournalSpace. Hey, even the GreatestJournal owner gave a few weeks' notice that that he was no longer going to maintain or fix that site. So, no, I'm personally not too worried about LJ as an online entity.
Then again, I've been backing up my LJ since the Great Strikeout of 2007 (otherwise known as the LJ Userbase vs. Six Apart and the Warriors for Innocence lunatics), so it could be a case of, "Easy for you to say."